You mean they owe $440K of the $730K? Otherwise your underwater and dead in the water. Move on!
If this is the case, then you have some options.
First step is talking to the bank, with the owner, in taking over the payments. This may help the owner get something out of the deal (credit save, keys for cash if in foreclosure). Or buying it from the owner outright, but you need to get a loan. First option is cheaper.
Once secured in your name, then you can do the lease option and hope to make a good profit in a couple of years, with a good down upfront to play with. Be prepared to put that back come decision time. You can realize a goodly profit, but it takes time.
You can also sell it for 70% of ARV ($486.5k) which gives you a little over $40K, minus costs or God forbid extensive repairs. Use that money to move on to your venture.
Or, sell immediately, after repairs, for a little under recent solds ($600K) for bigger payout of over $160K, minus costs. It may take some months, so be prepared to pay out-of-pocket. Winter is soon approaching, too.