Sorry I got to this thread so late, lots of great answers here though and good information. I thought I'd add a few things as I was --like you-- first inspired to play guitar by a simple (doom) metal riff, and all I had was an acoustic guitar.
Problem is, I have really bad dexterity, and the only instrument I have any experience with is a snare drum (which I haven't played in probably two decades). I have no idea if I would seriously be able to play metal music at all.
If you have "bad dexterity" it's probably just from a lack of practice. You will get there in time. I recommend starting out with a drop tuning like drop D (not to be confused with dropped d). The reason is because then you can play power cords with only one finger and that tuning is fairly forgiving if you make mistakes. I had a friend though who recommended not doing this due to it making scales harder to learn.. I happen to disagree. If you start in that tuning then you can always go backwards. Just keep in mind it changes the scales.
Also, I noticed that the kinds of guitars metal musicians use don't exactly look like mine. I think its one of those 'acoustic guitars'
This may be a problem.. I was in the same boat and tried to play metal on an acoustic (even trying tricks like using acoustic electric pickups); it's almost impossible to get a true metal sound on an acoustic guitar. Some metal uses intros and you can certainly play the same progressions/scales etc, but the sound will never be quite right for heavy distortion (other than black metal).
In fact, even among electric guitars, not all are cut out for the job. There are certain models/brands that are well known for being metal guitars such as: ESP, Ibanez, Gibson (Les Paul and flying V)
There are also specific pickup types such as active and humbucker that you can opt for which make huge differences in terms of sound.
EMG active pickups on an ESP guitar (for example) give an extremely unique and inherently apt sound for metal. You will probably find it a lot easier to do things such as pinch harmonics, tapping, fast melodic scales with active pickups.
Problem is, I don't know anything about guitars (other than the basics of how to actually operate one). I don't know what they cost, or what kind I would have to get. I'm not even 100% sure what kind of guitar mine is to be honest. I found some video online of a guy who managed to somehow play metal on a cheap 'box' guitar he bought for 'forty quid' (w/e that means).
I'd recommend starting out with a used Epiphone Les Paul. They can be had for pretty cheap and will get you started.
What does it actually take to seriously play metal music?
Time, practice, and dedication to the art (along with basic equipment and perhaps a drum machine to help keep beat at first).
What kind of skills do you need?
Music theory and guitar techniques for metal couldn't hurt. I tend to like the harmonic minor scales for writing metal. I've read knowing minor and exotic scales helps a lot. Some people use half tone, dimished, melodic, and chromatic scales. You kind of need an ear for how it's supposed to sound (which comes with time and a love for metal). The more you've listened to it, the easier it is to write.
(search online learning scales)
Palmetto, staccato, tremolo, palm muting, pinch harmonics, taping, chromatic and shifting key signatures are all techniques frequently used and the more you can add to your arsenal, the better. \m/
What kind of equipment do you need?
How much should I expect to pay for said equipment?
Picks 5$ - Heavy picks like Jazz III Dunlop help a lot for fast accurate picking IMO
transistor AMP (tube amps aren't good for metal unless you pay a lot IMO) - 100$ .. get something like a peavy (with high gain control) or a line 6 (has lots of built in distortion for metal and other genres). You don't need to spend a lot.
Can an amateur who's just learning even play metal from the start?
Absolutely, I just recommend starting with something like doom metal, which is usually easier to play off the bat than something like technical death metal.
I would also recommend getting a decent drum machine or something like Fruity Loops and make or use other people's beats to play along with. It will help keep you inspired and prevent boredom during tedious initial learning curve. Beats will also give you ideas for your own riffs eventually.
Do you need decent dexterity to even play a guitar?
Somewhat, I think you'll be surprised how quickly you pick it up, if you have a passion for it. It might encourage you to start out in drop D because those bottom cords are really easy and have a pretty crunchy sound when palm muted.