RE: First build ideas?
KIt building can be fun. That often outweighs the price difference that may or may not exist between kits and prebuilt models.
You mentioned a 20 size engine that needs a home. The SIG Wonder might be a good choice. I think I've seen a build thread on it and from all reports it is a quick and maneuverable sport plane.
There are 3 trainer kits listed at Tower that a 20 is in the recommended range: Goldberg Eaglet 50, Great Planes PT-20, SIG Seniorita. Even though you already know how to fly, trainer kits are good for learning to build. Since they are trainers I "think" they're more forgiving of newbie mistakes than some of the more advanced models.
In scale models, Great Planes has the T Craft 20 and a 20 size Cub. House of Balsa has an AT-6 and a P-51.
Everything mentioned in this thread is available through the Tower site. There are probably other sources for small kits but these are the ones that were fairly easy to find.