As a starting point, I use DaVinci Resolve. It’s a great editing program that is used by professionals. Plus, it’s free.
If you’re having trouble with getting the prequels, there are plenty of fan editors here who have made preservations of the movies. I’m currently using Schormann’s HDTV preservations for my fan edits, so if you want a recommendation, I’d give Schormann13 a PM.
Keep in mind, though, that the best route is to own the prequels on blu-ray and get transfers from the official releases, usually by pulling out a blu-ray drive and ripping the movies. However, I know how inconvenient that is. I only managed to get a blu-ray drive because it was my birthday. But it is the most legal option to go for. Plus, it makes it easier to get accepted into fanedit.org, which scores a few brownie points. If you have money you’re willing to spend, it’s worth it.
Schormann13 isn’t a member here