E36 M3 Buying Guide.

Rear Shocker top mounts
They wear out, check by lifting car up and down via rear arch, listen for knock, don't be confused with the half shafts catching though, you can remove the inside trim and bounce the car looking for excessive movement in the rubbers, but for 15 each ish, just get a set, various companies do stiffer ones, but beware as it does make a difference to the ride, you can hear more road nice through the car for example.

An obvious one. Evos have twin vanos, problems usually show round 45,000 Miles, but cars advertised as having a new one are not always a good thing. The usual issue is leaking solenoid seals, a good independent dealer can replace these for circa 250

Rear Arches
Check the rear arches for signs of rust, as they hold mud and rot from the inside out, especially if mud and wet gets behind the plastic covers and stills on the inside of the wheel arches.
You can buy arch sections for about 80.00 but cost me 500 to have them fitted.

Rear box over 500 and Cats very expensive, they rust at front and rear of the cat mount, where they mount to exhaust pipe, check for blows, also on the Evo there is a flex piece on one of the manifold down pipes that can crack / leak, and the manifolds are 100 - 150 second hand and up to 800 new so quite a hike!

My E36 Buying Guide.
For a good set of Michelin Pilot Sports, They handle best with these as were fitted std IMHO, you're looking at 600

Side Mouldings'
The M Sport mouldings' have metal inside them, these rust and the clips break off, ensure all mouldings' are close to the body of the car.

BMW's aren't usually too bad for rust, and most M3's if your lucky have been looked after, if not, look for rust in the following places:
Edge of bonnet, rear arches, around the BMW badge, around the boot lock, behind the boot handle, inside the boot under the boot seals near the lights, around the jacking rubber points and on the two bottom corners of the boot edge where the drain holes are.

(Ensure the ac blows cold as the ac system is expensive to fix if there's a leak in the condenser unit, up to 500. If its too cold the ac doesn't run, this is a feature)

Window Moulding's
A simple job, but the side rubbers by the rear windows perish, makes the car look old)

Door Rubbers
If split, they are expensive to replace, as they could have been letting in moisture, check under the cill trim as you get in the car, as the holes where the plastic trim fits to the cill can trap water and rust.

Electric Windows
Check windows go up automatically, if not the units that control the window stop may have gone, which is a pain as they wont close unless you hold down the switch, and full closure alarms wont b able to close the window, nor can you on the key.

Most M3's from 94 onwards have chip encoded keys that have to be programmed to match the car, so if you only have one key than can be expensive, just think cost of key + at least an hours BMW labour to code it up. (used to be an hour don't know now)

If they have diamond cut alloys most reburbisher's will not touch them, as the lacquer comes straight off again, and also if they are damaged BMW charge a lot for a new one. BMAutosport do a good selection of refurbished alloys usually with tyres, between 500 - 700, don't skimp on cheap wheels as they set the whole car off imo.

Front end
Check the bonnet catches are not to new, as these often get replaced on front end shunts. Check inner wing seam welds, should be neat and match each other, make sure all original BMW stickers ore on the front slam panel, and inner wings. Make sure bumper lines up, after shunts they can be loose or uneven.

Gear Box
Watch the 2nd gear syncro, they can get worn when previous owners have been snatching 2nd gear in the 0-60 dash.

A clutch will cost you a good 500 to fit, if the car has between 75K and 100K most are ready for a clutch

Hydraulic Clutch Pipe
As the cars get older the hydraulic clutch pipe can expand, meaning the clutch can drag, a new pipe, or a braded updated pipe from ebay or other websites is often a cheap fix.

Trailing Arm Rubbers
The rubbers on the front and rear trailing arms tend to need replacing circa 75K Get a garage to check for you, you can usually tell if it sides about a bit to much at the rear under heavy acceleration or deceleration.

Lambda Sensors
They burn out round 60K and there are 2 of them.

SMG Gearbox
Check oil levels for the hydraulic system, as if its low it will pop out of gear, can suddenly select neutral when driving and give the occasional error code on the dash. when warm get the car in sport mode (not manual) and drive it hard through the gear range, ensure the clutch doesn't slip.

Interior Trim and Equipment
Check for standard equipment, as a BMW cd changer connection lead if the car doesn't have one is a good 50.00 and a new cd unit can be 300. Ebay is obviously a good place to get things, but the e36 cd changers are rare. Look for signs of paintwork, usual spots are round edge of windscreen rubbers, under the rear lights on the rear 1/4 where the 1/4 meets the rear panel, and inside the fuel filler area.

Exhaust Note
None standard exhaust tend to be too noisy, like the scorpion type as they drown at motorway cruising speeds, Thornely motor sport do some good performance bits I believe.

Most of this stuff is obvious, but I've learned the hard way Hope its of some use, if you want some more info, feel free to email me.