the easy way to find out which gear ratio you want is to try different combinations on your current geared setup and find a sweet spot. most off-roaders start with roughly 2:1 gear ratio (but with 26" tires) and adjust up or down according to their typical riding terrain. ultimately you'll have to make the decision whether you want to be able to spin at 15mph or have a gear low enough for some serious off-roading.

A ss conversion kit is around $20 online. figure a cassette removal tool will run another 10 bucks or so (been a while since i looked).