Welcome to the site:
As far as what needs to be done, step by step:
For your car, you need a 98-01 3.2, or 3.5 engine. Fuel mileage is slightly better on the 3.2, but not by much. 3.5 has a few more horses.
Difficulty is an 8. You need automotive experience, a full compliment of hand (or air) tools, an engine hoist, jack stands, and a safe place to work on the car. If you've never done anything tougher than an air filter or spark plugs I recommend you find someone that has done major engine work to assist you.
Best place for used engines is car-part.com. Make sure they are complete drop outs, with all the accessories (a/c, p/s, starter, alternator, good harness, upper plenum, brackets, etc, also dont tell the junkyard your converting to a 3.2/5, b/c some are jerks and will void your warranty if you do tell). For a complete used engine with low miles and a GOOD warranty, between 1300-1500 for a complete plus shipping and taxes. Add about another $500 for conversion parts, and oil, fluids, clamps, misc stuff.
Both 3.2 and 3.5 are identical, except for the pistons and camshafts. Other than the 3.2 badge you cant tell them apart without tearing into them. Equally reliable.