The one bike rule is only in the Main build-off, this one you can enter as many as you like.
People are free to offer up prizes as they see fit, there are no 'official' prizes as it seems to make people take things too seriously, but nothing to stop people starting a prize thread and offering prizes.
Just bear in mind postage costs if the selected winner is overseas, the build-offs tend to get entrants from most continents, postage can be expensive if the prizes are bulky or heavy, and you may feel the pain in your wallet more than expected.
Luke.