I am a seven year pianist player, and after some thought, I have been considering playing electric organ (for the sole reason that they sound epic with the amount of instruments they play).
I have two questions:
Can I make the transition? And, if I can, what will be the challenges involved?
What kind of organ should I buy?
An important premise: my reply is about playing the electric organ as an organist in an organ trio or alone, to be able to produce a complete accompaniment for other musicians or for your right hand. If you simply want to play organ like a keyboardist of a rock band, my answer is not suitable for you.
Can I make the transition?
Yes, but do not expect to translate what you play on piano to the organ: it doesn't work. Playing the electric organ require a totally different approach respect to the piano, also if you decide to skip the usage of the pedalboard. I suggest you to use video tutorial by Tony Monaco (b3monaco.com). They are progressive and affordable. Just buy the first one ("Blues 101", if I well remember)and see what is the impact on you.
And, if I can, what will be the challenges involved?
Thinking like an octopus is the biggest challenge! Independence between the left and the right hand (and your left foot, if used) is the biggest challenge. What is great, apart from the sound, the satisfaction, etc., is that those abilities are realy usefull for the piano too. Other challenges: to be really confident with harmony - and if you are not, this is the right occasion! - and...what else?...yes, to learn bass lines both as notes and as "groove", and it has much to do with the independence. Other challnges (at least for me): open the right hand to increase the range of notes that you can take, to be natural in playing chords in several positions...and dozens of other things that make the organ unique!
What kind of organ should I buy?
In case all the previous answers make sense to you (refer to my premise), you should buy a console-like organ. The experts refer to them as "clones", assuming that Hammond console is the real thing ;-) and they are right! From this point of view, you are very lucky: clones are now affordable and they really sound like a real Hammond organ with tiny differences that only experts can hear. Only few years ago, the market situation was different. Some models, random sorted: Nord C2 (by Clavia); Crumar Mojo (the cheapest but maybe the best one in terms of equipment/price; it comes with very nice elec. pianos too); Viscount The Legend (really new, I don't know if it is already on the market. Similar to Mojo); Hammond SK family (they are not worth the price, in my opinion); Key B Duo (the manufacturer is the same of Viscount Legend; very expensive, very heavy, very beautiful); Hamichord (out of production, used only. It has one of the best engines inside, the VB3, and contains a PC that can be [potentially] configured with different software);
What I don't recommend:
Well, I count 4 questions rather than two.
And you need to clarify what you mean by "electric organ". That label can be applied to organs with a motor for air supply (but that's basically every church or theatre or other pipe organ these days), or it can be applied to tonewheel organs (of the Hammond persuasion). But the former does not readily fit in living quarters (assuming you aren't Count Dracula in which case we are more talking about undead quarters anyway), and the latter does not play an "amount of instruments". And will still provide the challenge of finding enough room (if we are talking about a nice classic like the B3).
So you are likely either talking about some electronic organ with analog circuits, or some digital organ with sampled sounds.
Now the problem with sampled sounds is that people don't buy into mismatched visuals. If your controller looks like an accordion, they expect to hear an accordion. If your controller looks like a piano, they expect to hear a piano. If it looks like an organ, they expect to hear an organ.
As a male alto, this was pretty annoying to me until I got tired of people's unflexibility and gave them suitable visuals when performing.
If you are doing studio recordings, you don't need to bother with people's expectations (Frank Farian is a notable producer who put up various show/playback bands for stuff he had been singing himself: great and flexible voice but looks like an accountant).
So if you are indeed talking "amount of instruments" rather than "sampled organ sounds only", you are more talking about a generic "keyboard controller" than an organ, something that's nicer for producing rather than performing. They start from a dime a dozen (though if you want to emulate organ, you'll have to invest into pedals as well).
The main question you need to ask yourself is why you want to play an "amount of instruments". Any digital instrument will usually be less under the control of its player than the original it is copying because it has only been sampled with a finite number of use cases. For producing music, 50 pretty good sampled instruments are definitely useful. In a band context, being able to play 2 actual instruments for real is usually valued more.
If we are putting the "amount of instruments" thing aside and really just go for organ, the main challenge is the difference in articulation. An organ does not really mind how hard you strike the keys but it is very much more particular about when you let go of a key. Because that does not just apply a dampener on an already decaying note but cuts the note off from full to nothing. So legato play and fingerings are a completely different issue from what you do on the piano.
Also if you are playing actual organ music, you'll need to work the pedals. That's another challenge quite outside of the piano experience.
There are different kinds of organ and organ music.
Do you want to play...
Church services on a pipe organ? The works of J.S. Bach, Cesar Franck and Olivier Messiaen? Gospel music in church on a Hammond B3 with a Leslie? A song like Green Onions? A song like 99 Tears? Jazz on the Hammond organ? Show tunes on the Mighty WurliTzer at Radio City Music Hall?
For the first two, you don't really need an organ of your own. This may sound surprising but many of the greatest pipe organists never owned their own organ. What you need is access to an organ to practice every day, at a church or school.
For classical pipe organ, if you can play a Bach prelude and fugue and a Mozart or Beethoven sonata you are ready to take organ lessons.
Make, model and price are all facets that are not dealt with on this site.
The main difference will be the action of an organ compared with a piano. Organ keys are often simple on/off switches, somewhat like a doorbell push. So, your well honed piano playing feel won't work here. There's also no sustain as in a pedal which holds the notes' sound on.
Your best bet for home use will probably be a workstation, which will come equipped with lots of organ sounds; there are numerous good ones on the market. I have some Rolands, which do the job of an organ quite well. The feel is organ-like, and the sounds produced anything from cathedral to Hammond, et al. There are also many other instrument sounds to be had, as a bonus. If Hammond is to your liking, the leslie effect can be operated using a joystick or a pedal.
Pedals as found on a lot of organs are a different ball game. Available as one or two octave, often midi'd (?) are expensive, and tricky to play well for a pianist, as they work with feet, obviously. I took a set off an old home organ, and operate them with midi.
The upside of all this is the gear is transportable, unlike a Hammond ( I know, I used to play in a Hammond trio!) inexpensive, and can be played through headphones to the absolute delight of neighbours.
