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1 ) T5. Its a bloat fest of a sandbox.
2) It is not difficult, it just takes a mindset change; Traveller characters are middle aged with experience and skills. They are not Level 1. You also don't experience as well. Combat can be deadly.
3) Quite! But they are not all GDW editions. Classic and Mega are 2d6, TNE is d20 (not *that* d20). But there are d20, GURPS, HERO and other official versions. Mongoose 1e is close to Classic, Mongoose 2e is also, but a bit different.
4) Mongoose 1e is good and probably the better of the two in that it is more complete. 2e changes things a lot. The systems are solid, but Mongoose is Mongoose, so just stick with the rules and nothing else. All the supplements are compatible.
5) See 1. Complicated, a million pages, it's a sandbox and supposedly good at it, but not many can get through it.
6) No.
Personally, I, like a lot of people, would recommend either the Mongoose 1e version or the Cerepheus <sic> version, which is effectively Mongoose 1e without the licensing. Both are consistent versions of Classic, can use all the Classic supplements, so you're sorted.
Just bear in mind, Traveller is NOT d20 (unless you use the T20 version). So your D&D player will have issues unless they are willing to go in open minded and able to change their mind set. The rules are a lot simpler and less bloaty, but you won't level up, you won't get tougher, you won't get more hit points etc.
Hope that helps,