18:16 is way to easy, your better off going for a rear 13t or 14t. Rims, there are only one type of 24 rim. The Ryhno light, they are strong, light and cheap on Chain Reaction at the mo they have 17% off the 32h rims. Tyre choice is very important, if you go for some of the maxxis tyres the bike will feel very slow and heavy. It will loose all of the amusing handling of a 24. The tiogas however have rubbish grip, they are god awfull on anything but dry concrete. Apart from the race bmx tyres like the Tioga comp X, which is what I run. It is a street tyre basicly and dosen't really cut it on the natural stuff. Another possibility is the DMR moto digga, but they tend to have very thin, weak sidewalls so punctures may be a problem. Hope that helps.