Hi Simon,
Very little between the 2.2 and the 3.0 other than the 60ish bhp. I would personally by on condition first, look for a decent service history/record. Most Z3's are getting to that age where most things will need doing. The youngest are over 13 years old, up to over 20 years for the early ones.
Just check for all the things you would check for on a 15-20 year old car. All suspension components, bushes, shocks and so on. Engine cooling rubber hoses, and the air intake hoses are prone to perishing at this age.
Worth checking for water inside the car as there are several known weak points other than the hood. The foam under the carpets in the cabin can hold litres of water without feeling damp to the touch. Check the battery box for water from leaks in the 3rd brake light and aerial grommet.
If you have a code reader take it along and see if there is anything stored. CAM sensors are quite common to go on the 6 pots. Obviously should be no lights on the dash - but be wary that a previous owner may have removed or covered them to hide a fault (sadly very common on the Z3). The engine should idle smoothly and pull consistently throughout the rev range.
There aren't that many jobs which can't be carried out by the average mechanic, worst problem will be rusty nuts and bolts. On the subject of rust! Generally confined to the outer skin panels, and is incredibly rare in the chassis itself. Finding decent second hand door sills is getting difficult, but all the panels are still available from BMW.
The majority of the parts on the Z3 are shared with other BMW models such as the E30 & E36 3 series, so are easily sourced and not overly expensive. GSF and Eurocarparts all stock almost all of the common stuff. We even have a couple of tame Z3 breakers on here, so really no shortage of parts. Sites like realoem.com will let you find all the part numbers you need and allow you to see which other models use the same part.
The biggest problem you will have is containing the silly grin on your face!
to the forum from West Cumbria. Nice one. The mechanical boffins will be along soon to advise on what's what. As for the best, can of worms 
it's obviously the 2.8 
