Well I wouldn't say there is more room to negotiate, but BMW should sort it for you as an approved used car, and you get 1 year warranty with an AUC car 2 years sometimes with the M. If they haven't you can use it to your advantage and have BMW to fall back on if something goes wrong. Like in my examples above.
Whereas if you buy privately you need to do your homework and offer appropriately. Basically is the service history up to scratch, the M needs to have the 1200 mile run in service anything over 100 miles over I would question or walk away. Then the car is serviced every 2 years (oil and brake fluid) regardless of milage done in that time or every 12k. Inspections are done at specific milage or around that, I think its 25k for the first and 50k for the Inspection II, but thats off the top of my head dont quote me.
Then there is tyres, brakes, suspension and overall cosmetic condition etc electronics what works? Seats can get tatty also trim is easily broken, Air conditioning - when was it last gassed and filters changed? does it work? Couple of engine checks how does it sound? The S54 naturally is tapperty so makes a metallic noise but if loud excessive then there may be a problem. Vanos can also be a problem, but cant think off the top of my head the symptoms. Also check exhaust fumes, should be grey not bluish grey, blue could mean there is an issue of running rich etc Check for hesitation, if you are at 1800rpm and hit the throttle does it instantly give you feedback if not then that shows it has the hesitation issue. Also valve adjustment needs to be done on the major services may be documented on whether it was altered or not all things you can use to your advantage.
Stolen from another thread think i summed most up:
• Engine
Check the service book for the initial, 1200-mile service. This is where the fluids are changed from the run-in spec to the normal spec, and if it wasn’t done then serious wear could result.
Curiously, the throttle system can go out of tune. Resetting it can improve throttle response and its really simple to do: turn the ignition on and wait for 10 seconds, then turn it off for 10 seconds, then turn it on again for 10, then start the engine.
The crankcase ventilation oil separator assembly can clog with sludge; in extreme cases this can lead to combustible vapour building up in the intake and going bang in a big way when you next start the car. The valve is fitted to many BMWs and is one of the most ‘popular’ replacement parts!
• Transmission
If there is a rattle when you switch the engine off, which doesn’t happen if you have the clutch down, then this is probably the clutch springs. If this is the case, then it’s fairly normal and nothing to worry about. Shift from first to second are notchy.
• Body and chassis
Your Z4 M should come with two keys, so check that they both operate the remote central locking. If not, then it is a main dealer job to get them reprogrammed.
Be aware that the strut braces can crack near the strut tops – it’s actually the weld on the body that breaks so replacement is expensive – so check this area carefully.
• Brakes and suspension
The road springs have been known to snap at 35-45k, so check the car sits level and there are no rattles from the corners when driving. Some owners fit uprated Eibach or H&R springs.
Creaking from the rear can be worn anti-roll bar bushes, but they are reasonably cheap to replace.
Search for ///M buying guides. There is extensive written guides
I think in a recent warranty thread people were being quoted about £800-1000 for an 06 car with average milage for a BMW warranty.