From iPhone 1st Gen to 3G change contract

Ok, heres the thing. I owned an iPhone up until the beginning of January when I lost it. My AT&T contract consists of a family plan, where only one line is an iPhone. I dont want to change/renew my contract with AT&T because I have less 8 months left to finish the initial 2-year contract. I know that the contract for the iPhone and the iPhone 3G are different and thats why theyd ask to renew the contract if getting a new iPhone. I was wondering if I could buy an iPhone 3G, disable 3G network data and use it keeping my old plan. Ive been looking for 1st/2nd Gen iPhone on eBay and such (this way I dont have to renew the contract, I confirmed), but I find them more expensive than buy a new one, and most of them wont be covered by apple anymore. D:
Help anyone?
Message was edited by: kuromi -
Macbook Mac OS X (10.5.6)