Zev,
I bought the kart from Dennis (2 posts up). It came with a Rotax on it. Originally, I wanted a Leopard, but after looking into it more, it seems the Rotaxs are very competitive and also enjoy a lower weight restriction.
I bought a used 3.5″x7.5″ trailer frame, then extended the deck out to approx. 5″x7.5″. Bought the frame for $175 plus another $100 in wood for the deck(wouldn’t think wood was so expensive).
Try searching denver.craigslist.com for used trailers. I posted a wanted ad on that site and thats how I found mine. If that doesn’t work, I’m pretty sure the heavy duty folding trailers from Harbor Freight are on sale now for $200. Add a wood deck and you are set.
As far as finding a used motor goes, I found that every used motor package was not much cheaper than buying new. Either way, I’d keep your eye on ebay. Every once and a while a used/new motor package will come up and sell for a good price. Also search the web for some kart classifieds, as there are a ton out there.
Are you going to practice with the KT100 on there or wait for a new motor?