The .40 might be enough engine for your training purposes, but that's about it. You're better off getting a .46, you'll be happy you did. There is one guy in my club who has the Evolution Trainer System and he hated it, and so did the instructors. He's flown with it a few days and already replaced it. They are broken in at the factory which is nice, but their performance tends to be lacking. The Avistar is a great plane; it's a step up (a little more capable) than a basic trainer due to it's semi-symmetrical wing. The Spektrum DX-7 is a great radio. You will need 4 servos, but they don't have to be digital. You are making a wise decision by not getting an RTF kit.

You will want a bunch of props in your field box, probably 11x5 or 11x6 (if you get a .46, otherwise check the manual of the .40). You'll need a glow ignitor, fuel, hand-crank fuel pump, either an electric starter and 12V battery or a chicken stick, depending on how fussy your engine is (I have an OS46AX, it starts every time with a stick. That is what I recommend for an engine - more expensive but way worth it!). You should use after-run oil in your engine at the end of the day. That's about all you need, bare bones, to get started. It's also a good idea to have a tether system for your plane for tuning purposes. A dog leash and long screwdriver works. I use a dog tie-down; it's like a huge corkscrew that screws into the ground.

Good luck, it's a LOT of fun!