First off, hello! I'm new here too, so I'm not an expert on this by any means-- got my motorized bike already built but brand new for super cheap, then rebuilt it for knowledge/fun/
of mind. But I do have a lot of experience with bicycles. Durability of a particular bike depends on what you plan to do with it. If you're putting around at 20mph, then sure, a cheap cruiser with no suspension is fine. If you're going to put a shift kit on it, and want to go 45, 50mph... then I'd look at a good mountain bike frame, maybe even get thicker gauge spokes and upgrade the hubs.
Pretty much everything you need to know is on the Classics board. I'd order a kit online after searching on Craigslist for kits/bikes-- my personal choice would be a single-speed cruiser. I got my 80/66cc HT cruiser with a 2.5L tank, brake/turn/head-lights with generator, disc brakes, and front suspension... all for the cost of a kit before shipping. He even painted the whole thing to match! =)
After you get your bike set up, break it in a bit and read the plugs (Classic board) and make adjustments as needed.
Protip; if you get a bike someone else built, don't be hasty. CHECK EVERYTHING. You don't want to get excited and take it down the road, only to have the rear brake cable slip and then have the poorly-adjusted front disc pads lock up... then realize that the fuel line, petcock, and gas cap are leaky, the engine mount is wobbly, and the tensioner is about to slide over and destroy your spokes. In my case, it was put together but never adjusted, never ridden. Either way, just know your bike before you ride.