What Kind Of Dirtbike Should I purchase?

http://ww2.cycletrader.com/browse look on here, all kids of bikes. You won't find much at all for 700 bucks, If you do it will most likel;y need a LOT of repairs. Save up and get a decent bike.

I noticed you said that you like to go fast, but you didn't put how much experience you have. I like going fast on my dirt bike, but I've been riding a long time. If you are just starting out, (and I assume you are) look in your local classifieds and find the cheapest, oldest 125 that still works. That way you don't spend a bundle of $$ and you don't have to worry about the wipeouts that you undoubtedly will take while learning. Start saving now, and once you've been riding for a couple years, get a 250...then you'll really be able to get going fast. Just wear a helmet, MX boots and for the first while I would even suggest kneepads. Have fun and don't break your ankles. Oh....and just to let you know. If you get a fast bike (like a 250) without experience, you will still go fast......in the ambulance ride to the hospital.

Yeah definately a 125 if you're 15. I'm 23 and a 250 is a little too much for me.

I started out on a '87 Suzuki 125 and it was great, it ran great and i bought it for 650.00, now that I'm older i have a Yamaha YZ250 which i love but it cost about 6,200.00 but i would have never learned to ride if it wasn't for that old bike, i actually miss it.

get some training and buy a rm125 for motocross wear a helmet and leathers and boots

I'd say get a Sanchez. And if you're lucky, you just find one/steel one.