I just got back from skiing, finally got around to learning how to parallel ski! Also picked up some new steel toe boots!
I agree with the take your time and make it nice suggestion. Maybe you want to make some sketches, and try to get a cost estimate so you don't run out of cash half way through.
Also, I like the style mini bike bighead posted, I have wanted to make one and gear it real low, like 15mph, then throw in an old 8hp or so Briggs or tech flatty, then take it on some trails!
As for the seat, you can make one very cheaply, get some good foam, plywood, t-nuts, staple gun, and of corse some vinyl from a fabric store, I am making a seat right now, got a 6th by 3ft chunk of heavy duty marine vinyl for about six bucks. Go to the DIYGK home page, some where there is a guide to making a simple seat, of corse you can make it more complicated if you want.