ok. bike buying --> bang for your buck: go to bike shop and get them to set you up in something used. talk to them and they'll recommend something appropriate for your use and price range (and it'll be twice as nice as anything you buy new for the same price).
shop around at different shops, and if you don't find anything there TALK TO THEM TO LEARN BRANDS AND FORM OPINIONS. Then you'll be properly educated to pursue deals online/ craigslist/ pinkbike, wherever.
also, if you're looking to build up your leg muscles I would definitely look into getting a fixed gear (please, no "anti-hipster hate" here). Put brake(s) on it so you won't wreck yourself on a car, or your legs from skidding. The resistance and constant pedaling motion will build your legs like no other freewheel bike can, and will increase "suppleness" (haha...just google fixed gear benefits).
anyways, that's my 2 cents. good luck!