I want to practice guitar with a drum machine instead of a metronome.
I want something that has a lot of pre set rhythms but also has some flexibility if required.
What features should I be looking for?
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Almost anything sold as a drum machine today will meet your stated needs.
As you know, a metronome just clicks at a regular interval. Sometimes it can use a different click for the first beat of each bar, but any more than that and it's more than a metronome. So let's look at what else you might look for:
The least flexible options will just have some preset rhythm loops. You get this in many consumer-level keyboards, and in some guitar multi-FX pedals. This can be a very effective and convenient "metronome on steroids".
More flexible, is the ability to program your own patterns. The oldest drum machines had no presets so you had to make your own patterns. Most programmable machines you can buy new today will also have presets, although there are exceptions. Check.
As well as the ability to program patterns, you should think about the interface used to program the patterns. Some machines have a difficult interface with too few buttons; like programming a digital watch. Other machines are very easy to program. Read reviews.
Ability to play live
You might want to play live "drums" using pads on your machine. Some machines have big pressure-sensitive pads that can be played like an instrument. Other machines have tiny buttons that make it impractical to play live.
Ability to save patterns
Some very old/simple machines forget the pattern when you switch off. There are hardly any like this on the market any more. What does vary is how many patterns you can save. Check.
It's common to have a fill pattern every 4 or 8 bars. It's also common to be able to trigger a fill by hitting a button. You might value this.
Most drum machines let you program a sequence of patterns into a song. "8 bars of pattern 15, then 8 bars of pattern 16..." etc. This is good for performance; perhaps not so good as a practice tool.
Some machines let you program a song, with elements of real-time control. "8 bars of pattern 15, then keep playing pattern 16 until I tap the footswitch..." etc,
This can be good for practice. For example, if you have different beats for the verse and chorus of a song, but you're still learning and making mistakes. You can practice the verse against pattern A. If you go wrong, wait for the next bar and keep playing. Tap the footswitch when you're ready to move on to the chorus.
How it sounds
It's plausible that the actual sound isn't all that important, if you want the machine purely as a practice aid. For practice, it doesn't matter whether a beat sounds realistic or artificial. Indeed the earliest machines were intended only for practice, and I think the designers were surprised when they began to hear their sounds on records!
Still, you want sounds you'll enjoy. So do take this into account.
I think if you avoid machines that are obviously intended as toys or novelties; and make sure you have something digital that has pads big enough for a thumb, you'll find that the other features are going to be present -- but do check.
Second-hand is a good way to go. Machine by Boss, Roland, Korg will all do the job. Zoom machines are cheap and also very capable.
I think what matches your (short) description best is the Beat Buddy.
I've seen it at a jam and it was really fun testing it, though I have to say I could only test it for like 20 mins. But the reviews at e.g. Thomann are very positive as well.
It's much more fun that the usual beat machines I had so far, at least.