I guess it would be possible with a parallel port modem. I'm not sure if an ethernet solution exists at all.
Yes, the 68000 can be changed with 68010, you can find some guides on the net to how to do it, but it's not that much of an improvement, and I heard it decreases stability of the system. To surf the net you should have something like 68030 which is only available through turbo boards.
By the way, you should be aware that each mod/ugrade you do will decrease compatibility with the old software (games).
