Buying a complete pre-used running motorized bike may seriously be a good option. In my experience it can actually cost less than installing a brand new motor kit on to your existing bike too.. Just be patient and wait for a good deal, then jump on it once you see one. Maybe for your situation it would be easier to just buy a complete bike. That way you can do little work on it here and there as it needs it, rather than having to do the full build right at the beginning.