Do some research, don't depend on others to specifically tell you everything.
Fuel Consumption
In Town: 10-12L/100km depending on driving style. Mine is about 10.5, and i drive relatively sedately with small amounts of highway in between.
Highway: 7-8.5L/100km depending on speed and consistency.
When Hammered: A lot.
Make sure it has a Full Service History. A FSH basically just means oil changes, beyond that you don't really know, unless the previous owner can provide invoices of work done. Ideally oil changes should be done every 10 to 15 000, which is more often than the factory specified 25 000km.
See here for common oil and fluid leaks:
http://forum.e46fanatics.com/showthread.php?t=882732
The cooling system is generally a weak point due to many components being made of plastic that gets brittle over time. Many beleive that you should replace teh cooling system every 100 000km or so to prevent any potential failure - if teh car overheats, you can very quickly/easily warp the aluminium head. I would say expansion tank and cap replacement are necessary preventive measures. Other important components would be upper and lower radiator hoses, thermostat, water pump, coolant temp sensor, and the radiator sometimes fails at the plastic joins.
Here is some reading:
http://forums.bimmerforums.com/foru...-E46-Cooling-Guide-(Information-w-parts-list) (this covers belts and pulleys which need to be checked as well).
At that mileage, you'll want to do a full suspension inspection, because there could be worn bushings (e.g. lower front control arm bushings, rear trailing arm bushings) and the shocks might need replacement.
Check the brakes - discs and pads bought from a parts store like Midas or Goldwagen are reasonable and on part with other vehicles. This is just normal wear and tear though.
I would strongly consider doing teh VANOS seals - this will no doubt restore some low end torque, and make the engine run smoother. PM Moranor on this forum for more info.
As mentioned, the window mechanisms do tend to fail, its a common E46 problem, again due to the use of plastic which brakes over time. Repair as necessary - but you can check for ones that are failing - if the window winds down faster than it winds up, then its a sign of imminent failure.
If you buy it, I would basically do teh following immediately so taht you know it has been done
-oil and oil filter change (it one hasnt been done in teh last 10 000km)
-powersteering fluid change
-brake fluid flush
-coolant flush (NB!)
-gearbox oil change (if its a manual).
-air filter change
The above aren't all taht expensive and you can pretty much DIY all of it (except the OEM gearbox oil is quite pricey, but this one is optional i guess).
That's a good start for you. Remember - An e46 is only as reliable as the maintenance of its past, and I'm not talking about oil services, I'm talking about addressing all issues that arise over time.
Good luck.