Mitch is spot on in terms of getting a realistic repair estimate (and don't forget to add a contingency because no matter how much experience you have there will always be something that comes up unexpected and adds time and cost) is key.
Contractors are stretched nowadays especially the good ones. Very few will walk a property down with you before you have it under contract. You can write the contract that it is contingent on an inspection with a contractor and use his input to make or break the deal but you need to be in the ballpark when you start. I am not talking about a home inspection which usually says "get a professional opinion" which is what you should have gotten to start with.
The other option is to join a local REIG and see if you can walk a few properties with members that are doing them and compare notes. They can show you how to come up with a repair estimate for your area (which can vary from other areas)
Good luck in your journey