1. D16Z6 since it's a 1.6 liter vs 1.5 liter. But realistically, the horsepower ratings are so close that it's not a big deal one way or another.
2. I suppose the D16Z6, simply for the fact that the correct ECU (P28) is more readily available than the JDM P08. The P08 also looks for a 1-wire O2 sensor instead of a 4-wire O2 sensor.
3. Neither would requre a kit, as long as you're referring to the '92-'95 JDM D15B vtec engine. Stock mounts, shiftlinkage, axles, and even transmission will work with no problems. You can absolutely do it in a weekend, even without any previous experience, as long as you have the correct tools available. Someone who has done it before can do it in 3-5 hours easily, but don't expect it to go that quickly if you've never done it before.
4. Too difficult to answer. Depends on what is available in your area, from people you know, local classified ads, craigs list, internet forums, etc. Obviously there are far more D16Z6's in this country, because they were sold in cars here, so it's just the trouble of finding one. On the other hand, if you can't find one, you can almost always find one at
http://www.hmotorsonline.com/
5. Really hard to answer. Used engines can be hit or miss. I've seen some used engines that a fine, while others needed over sized bearing to work, because the crank was marred up. Really, the crankshaft is the only thing you'd have to worry about, since you're going to replace the other parts anyways. With remanufactured ones, most of the time you know you're getting a good running engine, but you're also going to pay more money for it. That's definitely a good thing, but since you're replacing so many parts, it might not be worth it when it comes to price.