Optimistically advertised at £11k for a 2002 model, so a non facelift. That price is facelift money, we're talking 2004/2005/2006. Although having a quick look on Autotrader for 70k mileage examples with FSH and what appears to be fairly decent condition and not tatted up with black grilles and M badges on the kidney grilles etc, they start at around £12k and only go up...
Let's say you offer £8.5k and he accepts, I'd say be prepared to spend a few grand if you intend on owning it for a few years and doing some miles. At 70k miles, it almost certainly hasn't had the common sensors/components fail/wear that then need replacing. On top of that, the usual running costs for servicing, brake lines, tyres, bushes, flex disc, VANOS etc. I've owned mine since 62k miles and am currently on 104k. I've been lucky in that my engine itself has been fault free all this time. Oil changes are always done ahead of time and as far as I can see, previous owners have kept to that schedule too. It never needs a top-up either, so has no leaks. Supporting components have failed and needed replacing, but you expect that.
You've really got to REALLY want an E46 M3 to buy one now. It won't ever be a "oh maybe I'll try an E46 M3 next" purchase, it has to be because you want one from deep down, otherwise you'll be left eyes watery.
If you are happy with that, then read on
a knock from the rear drivers side suspension over bumps
Coupe or convertible? Most likely cracked spring. These crack like clockwork every couple of years on the coupes. Convertibles have different springs and sit a bit lower as well. Springs are cheap to replace with OEM ones, you could even DIY them, or if you have a decent local garage, just pay the hour's labour. Replace springs in pairs, like any suspension based component. Suplex (IIRC) springs are cheaper than OEM but are supposedly stronger with the same spring rate as OEM.
Some rust on the front drivers wing
They will all rust at this age. Replace wing with a new one. Aftermarket wings won't always be a 1:1 fit. A new wing cost for the driver side cost me £391, and a further £320 to paint and fit, along with cleaning and prepping the passenger side wing to prevent any rust on that one in the near future.
Bubbling of one wing mirror
Common if the car sits outside a lot on sunny days. The wing mirror base is aluminium which boils the paint from the inside causing the bubbling over time. A bodyshop can sand the paint down and respray them easily and fairly cheaply.
Oil service is cheap, but I bet the following service is an Inspection 2, which is at least £600. Buy the oil from Opie/ECP when deals are active (pretty much always?!) along with a Mann oil filter and call it a job jobbed. A garage should charge you an hour's labour to do an oil change unless you do it yourself and reset the indicator through the cluster hidden menu.
On top of that, I'd also consider cosmetic condition. Things like leather wear, bolsters especially as they can be DIY brought back to life, or be seen to by an expert. Steering wheel retrims cost £150 odd, new leather gaiters for the handbrake and gearbox to make them look fresh again have a cost to them, genuine BMW mats as the old ones will likely be very grubby as they are not items most people but a second thought to.
To be totally honest, I'm glad I've owned one now, engine is just brilliant and makes the ownership worth the efforts when you do let it go on the open roads. But if someone showed me a portal window that gave me a glimpse of the present day back when I was buying one to see just how much the costs pile on over 40k+ miles of driving, then I may have thought twice and bought something newer. It's not even a daily car, I mostly use it on weekends! But as said, now that I've owned one for a while, I'm glad I got it otherwise I'd never have been able to enjoy the engine any other way.