Main points to check are oil leaks from the cam cover gasket and the oil filter housing to block gasket, both go hard with age and repeated heat cycles, both DIY fixable. Roof motor/pump gets drowned if the roof drain gets blocked, always best to relocate the motor to the boot, a real pain to get it out but there are experts at relocation one here who can do it very quickly. I always recommend Alan, (Number5) on the Z4-Forum.com who is based in Bristol to do relocations he won't rip you off like some I could mention. You will need a decent code reader, bound to need one sooner or later with the amount of electronic modules these cars have, a good one is the Creator 310+ or the 410 model which is their latest one, circa £50 on ebay or Amazon. Try and buy a car with more than 1 key or its ging to cost you to get an additional key..
Do check out previous MOT history on any prospective purchase and look to see if advisories have been dealt with. Exhaust down pipe joint studs rust away and are a pain to replace as they are studs and awful to get out. Replaced mine with stainless bolts but took two days to get the old studs out.
Always good to plug a scanner in and scan the car your looking at and check for any warning lights when ignition switched on, airbag light should come on with key on and go out after a few seconds, if it remains on then there is a fault which the scanner should tell you what's at fault. Airbag modules, main one and satellite modules left and right are regular faults usually due to water ingress destroying them. Can be fixed with used replacements with acceptable part numbers that are compatible but they do need coding to the car with the Vin number. Z4-Forum members can do this and are spread across the UK so you may be near one if that problem ever arises.
If the car still has runflat tyres budget to get rid of them for normal tyres at the earliest opportunity.
Z4 as a daily will be fine.
Hope that assists your search.