Tacon 25 and Dynam/Detrum 40 amp esc on 4S 3000 (or higher capacity) will do great for general flying. I have this combo in a 40 size electric Pattern model.
Go to the Tacon 40 and a 60 amp ESC for heavy lifting and towing small sailplanes.
This will give the effective power of .60 glow and the Kadet Sr (with landing gear upgrade) can fly fine at 18 lb take-off weight on a weak .60
You can add even more power but there's not much point.
You might as well just go ahead and buy new... You'll have a warranty and you won't save much getting used.