Welcome.... start here for high level stuff.
https://zroadster.org/threads/z3-buying-guide-video.46189/
@Mike Fishwick is an authority as well.. just google him...........and then pull up a seat as you are in the right place for advice....whether you want it or not
The 2.2 is the sweetspot IMO.. best sound too
Always start with the basics .. Does the MOT history show any repeated advisories for the same issue (lack of care), Body and paint condition, Any engine lights on, weird noises, do all the buttons work, roof and rear window condition, check all carpets for damp, does it come with an inch thick folder of receipts and history..100k miles is not high! is it leaking oil.. actually forget that its a BMW.
As with all 20 year plus cars rust is your main enemy.. they go at the front and rear of the outer sills most often (easy fix but budget it in)... and also the rear wings. Bushings and suspension are likely to be shot if original.