1. What is the difference between a Handi and an Ultra? Is it just stocks and cosmetics?
No difference other than the stock, barrel may be marked Ultra, but that may happen on some Handi barrels too.
2. Am I restricted to only using varmint/bull barrels as extras? That doesn't give me much to choose from.
Depending on when your frame was made, you can have any barrel fitted except the 10ga and 12ga USH, if it was made prior to 2000 there are several exceptions that are spelled out in the FAQs build year info.
3. Can other barrels be fitted perhaps with a different forend? Can other forends be purchased for other barrel contours? There seems to be a number of different barrel contours depending on caliber and other factors.
Forends all have the same barrel channel except for a few shotgun forends.
Tim