Also, a few tips...
No service history, no sale - period. It's an expensive motor to rebuild - you simply do not want one that's had a palookah beat the snot out of it and not service it regularly. Confirm a claimed recent service by checking the air filter - if it's filthy, RUN, because you have no idea what else the guy is lying about as well.
Check the oil cap for sludging - if it has any significant deposits, walk away.
Ask the guy what oil he used outside of motor plan - if the reply is anything other than Castrol TWS or Royal Purple, walk away - Edge Sport 10W60 is also an option, but ultimately needs shorter service intervals than TWS.
Check for knocking from suspension components in the front - control arm bushings are weak and wear out quickly. Same goes for rear trailing arm bushings.
Gearbox is notchy, but should be smooth and have a tight shift pattern. If there's slop on the lever, you're in for shift bushings and possibly the linkage.
If any of the gears grind the synchromesh or pop out (3rd and 4th especially), you know it's been shifted like a numpty and most likely raced like a hooligan. Not a deal killer if the car's been fastidiously maintained otherwise, but prepare for a hefty bill to recondition the box - negotiate the price accordingly.
Lift up the carpets around the rear struts in the boot and inspect - a lot of hard launches or sideways action will cause the subframe to tear. Bonus if it has already been reinforced by BMW (another unofficial recall point locally) but an aftermarket "patch job" would need to be assessed by a good bodyshop.
Check for accident damage or resprays - if the guy tries to hide or deny it, steer clear.
Also, do the necessary legwork - get the car to an expert and have them do a full set of diagnostics, check motorplan for comments, etc. Check performance - Vanos isn't particularly problematic on the S54, but can still fail - same goes for limp mode issues, busted cam sensors, O2's, or MAFs.. If you're not familiar with how an M3 should feel and run, Get the car dyno'ed for peace of mind.. If it makes 175 kW+ at the wheels in Gauteng with no weird dips in the torque curve, chances are she's healthy with no wonky sensors or whatever.
And bear in mind that unless there's a very good reason like a kid on the way or an upgrade to a new M3 or Z4M, C63 or a similar car, there isn't really a good reason for someone to sell an M3 - unless they're facing repair bills or an imminent failure, and want to make it someone else's problem. So while it seems like a nasty thing to say, always treat an M3 sale with suspicion.
Think about it - they're not terribly heavy on fuel, very light if driven sensibly in fact, so unless the guy has a new job requiring 3000+ km a month of travel, that's a bullsh*t excuse.
If he complains about maintenance costs, you already have an indication that he may have cut corners in the past.. Etc etc.
Also check for worn brakes and an old clutch - negotiate your price based on these points, as both are expensive items for you to replace.
Hope that helps.
