What you should do--what real estate investors do--is approach it like this:
First, look for so-called "bread and butter" houses. Decent houses suitable for employed, stable, middle-class families. Not too upscale. And not shacks in poor parts of town. Good location, good houses. That's what you want to rehab.
Second, determine what the house would sell for in good, fixed-up condition, in 30 days. To determine that, ask one or more Realtors. (In return, you should consider using one of those Realtors to sell the house when completed. It's both a reward and a challenge: "You said you'd be able to sell it for $X within 30 days. Now, make it happen.")
Third, determine the repairs and rehab need on your selected house. Get a contractor (or two) in there to give you estimates. Include everything. Paint, carpet, windows (if needed), roof (if needed), HVAC (if needed), rehabbed baths and kitchens as appropriate, and so on.
Then use the magic formula: ARV (after repair value) *0.65 minus repair costs equals your MAO (maximum allowable offer). So, to work through some numbers, let's say a house would sell quickly for $60,000 when all fixed up. Multiply that by 0.65 (or, take 65% of that figure). That brings you down to $39,000. Then, subtract rehab costs. Let's say that's $20,000. So your MAO--the maximum you should spend--is $19,000. That formula accounts for a variety of transaction and holding costs, plus a profit. And if you use that formula, you should also be able to get a hard money loan (a loan based on the equity in the property, not on your credit), if you choose to go that way.
But remember the steps and do them in the right order: Find a bread-and-butter house, determine its after-repair value, determine the repair costs, then buy the house for no more than the MAO.
Hope that helps.