What should I be looking for when purchasing interior doors?
Depends what you are trying to achieve. Cheap egg box doors will fill the gap and keep the draft out, but look pretty lame.
Slightly posher egg box ones come in various styles of "panel" layout. Look a bit better. Still not much copy for a quality feel or for reducing sound transmission.
Engineered wood are quite a good all round option. Real hardwood faces, but stuck onto a MDF or similar man made core. Hence dimensionally stable, and unlikely to warp or crack. Much better noise isolation.
Real hardwood if you want to spend the money.
Mid priced fire doors can be quite a cost effective way of getting a dense heavy door.
Used doors from architectural salvage yards are also an option.
Finally something special's possible if you're determined.
See also: Category:Doors