It sounds like you're thinking the Super Cub is an easier plane than the nitro trainer. In fact, the opposite is true. Small, light planes get tossed around by the wind more and are harder to keep orientation on. The advantage of the foamy is durability for a flyer who wants to be self-taught.
Your $400 limit is pretty low, but doable if you do some shopping. I've seen flyable used trainers with engines and servos for under $150 in the classifieds and on Craigslist. You'll need a starter and battery, glow driver, fuel pump and something to put it all in. Keep an eye out for a guy selling an outfitted field box for $40-$50. The radio is the only place I wouldn't suggest buying used unless it's a unit less than a year old. Find out what brand the buddy boxes are at your club and get something compatible. Add in a flight battery, can of fuel, and sundries and that'll wrap up the $400 investment. The upshot is that you'll have something that flies well and will do basic aerobatics once flying in circles gets boring.