start with the most you can afford.

THen what the biggest thing you might want to make is.
Then what type of thing you are likely to make the most: practical stuff, models, toys etc

Then what materials you might want to use. (ie: sub 250c print temp or Above 250c print temp)

And at that point you can start putting a list of specs together and start looking for a printer that matches the specs as well as can be found :-)