Oh, and the same thing goes for keyboards: make sure it has a switch type that you can live with! There are various Cherry MX switch types that broadly fall into two groups: tactile, such as the MX Blue (audible click) and MX Brown (quiet click); and linear, including the MX Black (medium spring weight) and Red (light). Speaking personally, I never cared for their tactile switches in spite of generally liking clicky keyboards like the IBM Model M and like their linear keys much better, which some people say they do much better anyway. There are various other types you may find in the wild like MX Green (much the same as Blue but heavier), Silver (like Red but a higher actuation point: they'd drive me nuts) and so on. And then you have a whole slew of Cherry compatible switches from other manufacturers with various properties and qualities of their own.
Surprisingly for me, I ended up not only preferring their linear switches but in particular the lighter MX Reds: I thought a light spring weight would be awful based on experience from many years ago but it turns out what I specifically disliked wasn't the amount of resistance but the keyboard's twitchy nature, namely that actuation point thing. Sometimes an educated guess about what you'd prefer can be quite wrong, and sometimes on several counts, as it was for me. Then there's stuff like "Silent Reds" which I haven't tried but suspect I would dislike: standard Reds are quieter than Blues but I like their slightly clackity nature. Other people don't.
Layout is also important: some people love their tiny 60% or even 40% keyboards but I can't really live without the convenience of my arrows, function keys and editing keys not requiring a modifier key to be pressed at the same time so 75% is as small as I can go. ISO vs. ANSI, which mostly boils down to the position and size of the return key (large vertical thing for ISO, smaller horizontal bar for ANSI) and a couple of other differences. Again, some people have strong preferences one way or the other; I've never been able to get used to the ANSI return, for instance, even when it was the only game in town, whereas other people don't care.
Layout is also important for the key sets: generally speaking, a preference for ANSI will give you a lot more choice as it tends to be the default option, with ISO often being an add-on kit which may or may not include country-specific keys, so ISO-only will give you an alternative return, backspace, backslash and left-shift key whereas a UK set will be all of that as well as `/¬/¦, 2/", 3/£, 4/$/€, '@ and ~/#. Custom key sets may or may not cater for non-standard layouts (e.g. those reduced size keyboards) and in particular it's unlikely they'll have the additional legends (markings) required by specialist modifier keys, which may or may not be a problem.
Plus other aspects of construction, e.g. size of bezel, metal vs. plastic case and so on. It can be quite hard to get a feel for what they'd like to be live with unless you actually try them out for real, things like key-testers giving such a limited idea that you need other means of forming an impression. Something I've discovered to my cost, quite literally.
Don't just go for "a keyboard" with "a keyset" on the basis of how it looks: you might be happy with it but then again you might hate it!