Welcome Splitty
Wide body, only the preface lift 4pot the 1.9 M44 & M43 were narrow body, all other versions are wide body, with the face lift version getting some bumps on the top of the rear arches, so do have a look at both types of wide body, as personal taste can take you on a direction, there is more choice in engine on face lift models with 1.9, 2.0, 2.2, 2.8, 3.0 and 3.2//M flavours, pre face lift you have the choice of 1.8, 1.9. 4 pots and 2.8 & 3.2 //M six pots. All engines are bomb proof as long as they have been looked after, overheating is the main issue on a four pot, so water pump changes are a must. Six pots have the VANOS timing system,Mathis is prone to wear,mso avoid a rattling one, but also look for things as VANOS seals in service history as it is an indication they have been looked after changed, on all six pots oil service is key, look for regular oil services, avoid low mileage garage queens that have not had an oil chsng for more than six months, this us especially important on the 2.2 engine, as this is prone to sticking oil control rings and blocked CCV all which lead to burning copious amounts of oil,MA good engine,mbutbits needs looking after.
All cars share the same suspension except the //M and Sport versions which are lowered as standard, these cars have no camber adjustment on the rear, so you will see negative camber on the rear of a lowered car. A worn suspension system, including bushes can lead to a wayward drive, but they are easy fixes, to wide tyres on the front can lead to tramlining, original wheels were 15" steels and 16" or 17" alloys, tyres should be 205 to 225 on the front and 225 to 245 on the back depending on the model.
Look for extras such as roll bars, not all cars were fitted with them, wind deflectors and tonneau covers, if they are included it will save you £500 later on, it's the same with a hard top,msecond hsnd they can be £1000 with fitting kit, so a big bonus if included.
Not all cars had a hydraulic roof, early vehicles and dome later cars are manual, not all cars have a leather interior, some are cloth (avoid), some have heated seats, sports models have special sports seats, different from normal leather seats and similar to //M models, Individual models are characterised by special two tone leather interiors with matching leather trim to the door cards and the dash.
Mechanically a good car will drive like a brand new car, you may hear a slight wine from the diff, this is normal and you are sitting over it, an excessive wine could mean trouble. Gear boxes should be slick and tight, but can be not by during cold periods, oil change can remove this. Six pots suffer from selector pin wear, and four pots suffer from shifter bush wear,mbutbits otherwise robust, all have a DMF fitted, so a juddering clutch could indicate this is on its way out,
Check roof for good state of repair, no holes, check inside for mildew, mould or a musty smell as can indicate a leak, feel all carpets including boot for damp,man other sign of a leak, with the boot this is normally the boot light gasket.
Pull the boot carpet up and look for popped boot welds, not all Z3 are affected, and can be repaired, but best to walk away , remember theses cars are all over 10 years old now, so if it the weds have not popped they won't .
Although these cars are all over 10 years old they were built very well, a rusty example is a sign of neglect or accident damage, the paint work should look like new, apart from the standard stone chips.
My advice is to drive as many engine types as your budget can handle, once you've decided on engine, find the best well serviced example you can find, and mileage should not be a big factor, as said earlier they are built to last if looked after.
Lastly, I am sure that when you do find the one, a forum member close to you will gladly come along and give you his thoughts in person.
Happy hunting