Your choice of a mike pre-amp depends on how much you want to spend. Probably anything you find in Sweetwater's catalog will do you fine. If you're willing to spend more than $600.00 you can find some very nice equipment for recording. Live, probably you won't hear a major difference and quite possibly plugging the mike directly into the console should be fine.
You use a DI to take the guitar's signal straight out of the pickups to go into a console or digital sound card for recording. That way you can "re-amp" the guitar signal after recording. That is, take the recorded signal and run it back into the guitar amp so that you can mike and record the speaker output, possibly a multiple number times with different settings.
