Can i put a rebuilt engine in my 2002 explorer and do iIhave a choice as to what kind?

You can put just about any engine you want in it, as long as it fits your fresh trans. The kicker is the control system, PCM and sensors. If you get a wrecked truck with the engine you want it will have the pcm and all the wiring. Some salvage yards have the entire engine/control system available, but since you'll be paying some one else to do the swap you're probably better of to stick with a rebuilt 5.3. If you like the truck an engine is a lot cheaper than a car loan

Every swap I've attempted on a late model car has gone to crap if I tried to change the engine out with a different engine. Basically, I got tired of chasing problems and mis- matched components. The most reliable and trouble-free thing to do is to replace the engine with the same engine. No fuss - No muss.

Yes, the V6 4.0 has two cam chains, one in the front and one in the rear which requires the engine to come out to replace. It is expensive. My nepews are still driving my 96 and 99 explorer V6 but they came with the push rod version. Both are pushing 275K and still going strong. I would not go with anything "rebuilt" as they are only rebuilt up to the previous failure. Spend a little more and get a remanufacturered engine. Most come with something like a 100K warranty. You may or may not have to change some electronic components, check a reputable shop and they could advise.