I second building one yourself. It's not to difficult and you will know every nut and bolt. Keep an eye on craigs list for a suitable(comfortable) used bike. $450.00 should be plenty if you start with a used bike from your area and go with a basic 2-stroke kit. Be aware cost can go up quick for more quality components i.e; balanced motor, cnc rear hub/sprocket, performance carb and exhaust, heavy duty wheels...etc...etc..etc.
To start though read through this forum. There is a ton of info and if you can think of it, it probably has been discussed here.