A Boeing 737 type rating does cover you for all variants provided you do a conversions course which is just like an add on, maybe 3 days or something class and a sim session or 2 .... Not sure the exact requirements. So for example a guy rated on the 737 classic 3/4/500 would just have to do a little conversion training to fly the NG 8/900 i.e NG Differences course, bit of a non event really. The same works the other way i guess . Its more of a familirisation really.
I really cant undrestand why your getting hung up on these details at your stage.
Financially you will need the thick end of 60 > 100 grand.
Academically you need to be switched on and very motivated.
If your going to be put off by the looks of a tricky subject in a book, then its not for you.
Go get the medical and VISIT the schools.
Airbus ratings are generally more expensive from what i have seen.
Guys please consider, instructing,glider towing,para-dropping,aerial work etc etc before leaping into a jet.