You bought a bike without a stem? That's an interesting one. Are the handlebars just dangling off the front by their cables right now?
As for specific recommendations -- first you need to know the size you need. Not all seatposts are the same length or diameter.
Same goes for the stem -- there is threaded vs. unthreaded (the newer way), and the handlebar clamp diameter, as well as the head tube diameter. I'd ask the original owner if he has those dimensions available; otherwise, you're going to have to go measure (or get your LBS to measure for you). After that, you need to determine the proper length / rise stem you need to make the bike comfortable for you.
Once you have all that figured out you can begin to figure out which components you need.