If the main purpose of the welder is to be welding props, I would get a mig welder, and use 75/25 gas. You will be able to use it to fix things like a trailer as long as you get the metal clean and out of the wind. If not flux core or a stick welder is the way to go. Most fab shops use mig for production work, it's fast, and easy. If you want to weld aluminum there is no better choice than a tig. If you can weld with a torch, you can weld with a tig. Aluminum is tricky, for most people it makes more sense to hire it out.
you can get a spool gun for your mig that will run aluminum wire, but nothing beats a tig.
I have been a welder for years and have owned all of the above, I now only have a mig and a torch, I just couldn't justify keeping the others.
as for the props, most of them that I have bought or built use 1/8" wall 1"x1" square tubing, bigger projects require larger tubing. Based on the fact that you are asking these questions, I would recomend that you not attempt bigger props for a while.
Hope this helps