Hey Mate,
Since no-one else seems to have given you the courtesy of a reply, let me try.
In my experience, the air cooled KDX's don't make as much power as the later models, and so you may be better off with the 4.00 rather than the 4.50 (the wider tyre sucks more power).
As far as engine gaskets go, definately buy a head gasket, but for the rest of them a good quality silicon gasket goo such as Permatex will do nicely. You can even use it to seal the joint where the pipe goes into the head.
bearings - OEM or aftermarket? whichever is cheaper! shop around. Generally speaking, Kawasaki bearings aren't overly expensive, and sometimes you find them a bit cheaper!
Good luck, hope I've been of some help.