Yes, you can build with just the 3X rivet gun.
Yes, you will need to have the C-Frame for dimpling, but I never set a rivet using it.
When I bought my tool kit, I deleted some items that I already had, like files, tin snips, punches.
I would definitely get the air drill and a good compressor. You'll need the compressor for the riveting. I also would get the pneumatic sqeezer if it isn't in the standard kit. The money you spend on this, along with an adjustable set and various yokes will make your build SO much easier.
I highly recommend that you get a good drill press and band saw, along with the bench grinder. The one power tool I didn't have, but wished I did was a bench disc/belt sander.
Get the basic deburring tool that you twist. If you have a helper, and need another one, they are cheap. Same goes for an extra pair of cleco pliers.
I found during the build that the basic tool kit is just a starting point. You will need to buy more stuff as you go, especially when you get to the wiring. I bought some specialized tools for rivet removal, edge rolling, etc. and they were worth the price. You can never have too many tools!