A single din head unit is about the only "requirement". A stereo from crutchfields will include the special removal tool for the OEM unit.. a harness plug adapter.. and an antenna adapter. Crutchfields doesn't have the harness adapter for wiring directly off the stereo's speaker wires. There's is an rca plug, which will work fine "if" you make sure the head unit has 4v output. A Schosche (VW) one does work fine for speaker lead setup.

Those are the "basics".. a search on stereo will bring up plenty for the fine tuning on the install.