Buying an altimeter watch.
You must be able to read altitude without glasses
Test readability in bright daylight
You may have to calibrate the watch several times a day. In very cold weather. The function must be available very fast. Going through some menus is a no-go.
Keep in mind that calibrating is usually just a few meters correction. This must be fast. If you have to start from 0 then that is a no-go and error prone.
5 m steps must be distinguishable with ease. Difficult with watches with a dial. Digital is better.
1 m is even better but not mandatory. Of course, barometric changes are bigger than 1 m but you do not need to think what you should put in if altitude at your point is 2345.
In digital watches “Positive” (black on white) is better than “Negative” (White on black) [I am not so sure here, please test]
Battery life is all important! Changing a battery with gloves is quite difficult. Do not combine watch with GPS or similar.
Take one that does not need a battery change (solar) or have one where you can change battery yourself. Have spare battery.
Buttons must work with gloves, at least thin gloves.
Precision. This is difficult to evaluate yourself. Calibrate and change altitude fast, at least 1000 m. A good watch is precise to a least 5 m.
Must have a temperature compensation. Read manual. Watches that also show the temperature are usually compensated.
Must be readable in the dark
For real outdoor. Keep in mind that this is the most important instrument you have, in many situations more important than the compass.
You must be able to do all functions in -20 C in a storm but also in bright daylight.