If you are motivated now, then ride now. Problem is EVERYBODY and his uncle has been purchasing bicycles this year. The shops are literally sold out and $600 is smack dab in the middle of the range for everything being sold, so good luck. Look on Craigslist but expect to pay top dollar.
That said, a hybrid for now, from a local shop that can fit you, also have them make sure the wheels are decent as that's where you will have the most issues due to weight. Frames rarely fail, neither do seat posts. Sometimes the support rails of the seat as well as the original cheap pedals. The front wheel coming off is really only an issue of not correctly installing the quick release, as wheels usually do not come off.