Welcome back to the hobby. I did much the same thing and started back up again late last year.
I wouldn't think you'd need to thin it at all. I airbrush it right from the bottle with no thinning so I wouldn't think it would need thinning for hand brushing. Admittedly I haven't tried it myself as I always airbrush it.
Future Floor Polish is the modeler's do-all for clear coating.
See This Topic in the Techniques section for everything you ever wanted to know about using it. For a flat finish you can mix in Tamiya XF-21 Flat Base and it will cause Future to dry flat. About 4 parts Future to 1 part Flat Base works well for me.
I have also heard a lot of good things about Poly Scale flat. Haven't tried it myself, but I intend to.