1) I like .60 size trainers. They're more stable and give older trainees a little more time to react. Better in the wind also. Only downside is slightly more money
2) All the trainers I can think of have a one piece wing. Probably 6 feet long.
3) Rubber bands are pretty practical. Put a mark at the centerline of the wing at the leading edge and trailing edge. Make a similar mark on the fuselage saddle. Line them up when you put on the rubber bands.
4) The K&B was a classic low cost .40 glow engine. It will probably work fine. The new engines are even better however. If you buy a new one see if you can afford one with two needle valves (high speed and low speed) and a ball-bearing crankshaft. The are basically two tiers of glow engines. The low-cost engines have a single needle valve and a bushing-supported crank. The better engines have two needle valves and ball bearing supported cranks. Most major manufacturers supply both types. With OS, for example, their low-cost line is called "LA" and the better version is the "AX". A OS .46 LA may cost $90 while the OS .46 AX may cost $120. The more expensive engine will run more reliably and produce more power. Both will last a long time.
5)The Saito is an excellent engine and you can use it in your trainer. There is a slight risk that you could damage it in a crash, but if you get an instructor with buddy box, you really should not crash.