Am searching for E46 too, since most units are close to 10 years or more there tons of stuff that ay go wrong.
Should you plan to take up a loan, please have a quick look of the car, make a copy of registration card, try apply the loan first. Let's not waste the seller's time as I understand how frustrating it can be when so many potential viewer/buyers but none can afford it.
While the bank process your loan, search for a reliable and trust worthy mech to agree to scan and check th car and remember to agreed on the fee too.
A full scan engine, body work visual check, make sure no accident, flood, interior condition, and last take for a spin for drive behavior. A well taken care of care will drive like new, good alignment, solid feel, quiet, interior smell fresh, ets.
Do remember BMW are known for banyak masuk workshop even it's new so do prepare some cash and get ready to learn to be confortable lepaking at workshop.
On what to repairs and replace, please visit the e46 thread, tons of in for there.
Most reliable engine is M43 while N42 is something you need to stay away. M54 is the ultimate engine but demand TLC.
Good luck on your purchase.