You need to factor in the above. Obviously these cars are 12-14 years old now and most need some maintenance.
Preventative maintenance - rod bearings, Engine and Gearbox mounts, Engine Bolts are things that spring to mind as things that should be done. The mounts are rubber and one of the engine mounts lives behind the heat shield, most probably need doing at this age. There was a recent vid I watched on the E92 M3, similar mounts 2012 car had them done at the same time as the shells. They were completely shot 60k car obviously 6 years newer than the oldest Z.
Anyway if you get it all done at the same time its not much more roughly £1500 total. Can really transform the car too so worth doing.
Inspections can be expensive £550ish for INSI £850ish for INSII depends where you go. Many third party non specialist garages will tick and inspection and not do it, its not a standard service and the shims are usually checked for tolerance and I would be looking for evidence of this.
Generally service schedule should start with a run in, then 12k per oil service or 2 years, Inspections are usually done every 25-30k and are mileage dependent although many are done early. Many dont tend to get to the mileage so time is important. Even at 100k its only 7k per year if a 2006 plate. Lots of paper work is key, hopefully over than under serviced.
Other things to bare in mind are vanos although there aren't lots of vanos failures, diffs seem to becoming more common item that needs attention. Obvious stuff like tyres -500-1000 depending on preference. Brake and disks are around £600 an axel £1200 for both. Also corroded brake lines which are also common.
Same with suspension, im sure they will be fine but it will probably be tired. A lot of cars are lowered and can make the suspensions shocks leak. Springs are prone to break etc Mine leaked at 60k and had the rears replaced.
Generally people tend to buy and sell when maintenance needs doing so keep your eye out. If you can find one that has had the maintenance done then happy days but need to do all the above you could easily spend 3k which will ensure the car is good to go but you could possibly get into a better lower miler.
Cosmetically things to look for: the front bumper will probably have been painted at some point, they are low and prone to chips. Windscreens tend to get well chipped too again because its low, fangs on the front bumper tend to get damaged and the skirts tend to get road rash. Other things to look for, rubber between the drivers door and rear window. They tend to loose the coating and can look a bit unsightly. Interior wise common faults are wear on the drivers bolster and the seat belt can be slow to retract, easy to sort with some soapy water, generally just needs a clean. There have been a few with broken glove boxes, and the aluminium/piano black trims can get marked and scratched easily.
Depends on the budget. Ive seen 100k cars for sale for 16k one sold earlier in the year for 15k with all the above done. Fairly certain there was a car with 135k that was for sale for £14k. Obviously the sold prices are hard to know but say 10-15% wiggle room depending on the condition.
In all honesty these dont really have any big issues. As long as they have been serviced correctly and preventative maintenance is either about to be done or done they could easily soldier on. There are no chassis issues, no real rust issues (maybe the boot but not seen many Ms with corrosion issues) Engine is really strong for its calibre.
Common sense and a bit of knowledge could bag a great car for a good deal lower than the going rate. Non of the above is a deal breaker just worth knowing so its not a nasty surprise. Low milage cars are really sort after and can sell for up to 30k so you could bag one for half the price, they are so rare with only 550 cars originally coming to the UK that its a great opportunity in this day and age of monotonous vehicles to get a rare car not often seen for hot hatch money.