I’m in the US so some things may be different between my E46 and one you may buy down in upside down land. Also I have an M3 so some things (mainly engine issues) don’t apply if you go with a more standard model.
Overall, the E46 is a generally reliable car. Engines, trans, overall drive train is pretty good. However there are some pretty important things you should watch out for.
Window Regulators: They are shit and can fail meaning your windows will stop working. They will usually make a loud clicking sound when they are having problems so it is easy to tell.
Power Steering Pump: Less common but it can fail. If you hear a growling like sound on full lock stay away. Check your fluids.
Cooling" The water pump and other components can fail spectacularly leading to near instantaneous loss of coolant and overheating. Relatively rare but can be disastrous.
Tail Lamps: Recall was issued over the wiring and failures there
Rear Subframe Cracking: This is a biggy. BMW was sued over this and lost. Many E46 BMWs had their rear sub frame crack or tear essentially ruining the vehicle. Be aware of loud noises, scraping or clunking/creaking when driving over moderately rough roads. It is rare but something to watch out for.
Suspension Bushings Known to wear very quickly and near replacement
That is pretty much all of the known major problems. The M3 has VANOS issues that aren’t found on the lower models so don’t worry about that. All of those issues mentioned, other few minor ones, are known problems. I know it seems scary but get it checked out and once repaired (if necessary as not all cars have issues) they are excellent vehicles. Handle well and are fun to drive. Any car over ~140K km should be gone over with a fine tooth comb at an experience BMW shop or dealer before you buy.
Just note what you’re getting into here. You’re buying an old German luxury/sports car. They require care and maintenance. That care and maintenance, parts, ect is expensive. If you really don’t have much to spend on buying one, can only afford a high mileage one, or not to maintain it then don’t buy it. The E46 is the best 3 Series BMW ever made and one of their best cars of all time, if not the best, and they are incredibly rewarding. You do need to put in effort though.