Honestly you're more than prepared to set up a decent tank. For ferts in my low tech tank I use:
KNO3 (potassium nitrate)
KH2PO4 (monopotassium nitrate)
Swapped from EZ green although sometimes I use it since it has less nitrates than thrive. Tanks are all about personal preference, for 5g you're limited by stocking but you could do a showpiece betta or a small school of one of the smaller tetras. You could also turn it into a shrimp haven. Just find plants you like and get those, I suggest the marketplace section of these forums if you're looking for plants.