any sled that is going for 1k is probably going to be a 95 or older or something that has been beat to shit. 1500 os a more realistic figure that you will definitely be able to find a decent 500 or 600 liquid cooled sled by what ever make you want. im not going to be too biased, but definitely stay away from yamaha sleds.
most likely you would want a 2stroke bike, when your just trying to find a affordable bike to get into the sport, 4stroke will usually be out of the picture. unless you plan on getting something like a honda 230, the 4srokes have a limited life on the engine, especially the valves. you will still have to rebuild a 2stroke every now and again, but a piston and rings are alot chaper then sending your head out to have guides, valves, etc. done. most of the time that will run you about 500 spot, if the head is within spec.