You would use a hard money lender. and they will require roughly 15% down, reserves equal to at least 6 months of monthly interest payments and roughly 10% of your rehab budget. Whatever they say you need minimum, assume you have to add at least $20k-$30k because they also usually require you to pay for parts of the rehab first, then you are reimbursed in "draws" so if you have to redo the plumbing for $15k, that's coming out of your pocket first. Then they charge you for the draws. The closing costs are insane and they usually have origination points from 2% to 3% (2% to 3% of the loan as a fee upfront.. just because).
I think step 1 is hook up with a realtor that works in the area you're looking in and get an idea of how quickly things are selling. The key with any place is ... will it sell when you're finished. If properties are taking 6-7 months to sell.. it's probably best to stay away.
In regards to square foot for construction, it's really difficult to estimate. I would say for higher-end, expect to pay $200k or more. I am working on a project now that's not even a full "gut rehab" and it's going to cost us about $100k for a single family home (ARV of $370).
If you don't have the money for the stuff I mentioned in the first paragraph, start working on finding it!