I've dealt with quite a few 3d lumps. 70k, really all should be fine, 100k, you do need to be a tad bit more careful.
DEFINATE THINGS TO WATCH OUT FOR:-
Rough idle, and VAG diesel sound on startup
This could be the vibration damper control unit. A woodruff key on the bottom pulley comes loose and the pulley shakes causing rough idle and a noticeable rattle sound even when warm. DONT walk away, knock money off, but its quite easy to fix.
This could also be an injector on the way out. Whilst the part itself is only about £40, the labour can run in 20 times that figure, so you may as well get the lot replaced in one hit. IT IS POSSIBLE TO DO THIS YOURSELF IF YOU ARE CONFIDENT. However, there is a Haynes manual for the E46 3 series BUT IT DOES NOT COVER THE DIESEL ENGINE
I have seen this idling rough from injectors cured by a low tank flush through with some injector cleaner, you will also notice very poor fuel economy with this problem. Its your call whether you walk away from it or not. Also, IT COULD HAPPEN soon after you buy the car, unfortunately there is no way of knowing, it just happens. If this is the case a flush through will and should sort it.
Suspenion
The rear arms are prone to snapping (something to do with Porsche 911 beating torque), check that they have been replaced AS EARLY AS 70K. If not, make sure the car is sitting level and check for hairline fractures in the rear arms. Touring Springs have been known to snap as well, but your not buying a Touring.
Lastly, check all the electrics work, apart from that there are no other major faults.
If you take it our for a drive, the left stalk has a button on the end, flick through the options until you get to mpg, press and hold the button, it will reset, and you will get an idea of mpg. Range your driving style, but if you get less that 30mpg on a hard thrash, think twice.
Check to see how many mile are due to next service. It goes as follows for E46. Insp 1 (reasonably cheap), Oil, Insp 2 (expensive), Oil, Insp 1, etc, etc
Make sure the oil is changed. You should have NO smoke out of the back IMO, if you do, it will either need an oil change or air filter replacing. The airfilter can take an hour to change as all the engine accoustics need to be removed.
Lastly, and most annoyingly, urine smell from the AC/CC, change the carbon filter, but unfortunately this smell sometimes can not be helped. Leave the AC on to get rid of it.
HTH