First, if these are REOs, then you will not be able to assign the contracts, period. The seller's contracts/instructions will specifically say that assigning is NOT allowed.
Second, assuming that $700K is retail, the max that you should be paying for this is $490K, especially if you goal is to assign/wholesale.
Third, assuming that you get it for $550K and do a double-close for $700K, your estimated closing costs (based on 3% of the price) for the first (the buy) closing would be aprox $16,500. On the second (the sale) closing, it's $21,000. Total closing costs $37,500.
Raj