Andrew Brown,
You need to think about this in terms of what an investor buyer will want to pay for the property. This is predicated on two basic variables:
- What will it be worth in a fixed-up state? (After Repair Value, ARV)
Take an honest look at comps in the area of similar condition and age, recently sold. Just do your best, and keep it honest.
- How much money will it cost to get it to above fixed-up state. (Repair Costs)
This is likely your greatest opportunity for error, considering your lack of practice. Do your best, and try to get an opinion from some local contractors if you can.
Then subtract the Repair Costs from the ARV, and take 70% of that final number. That's what a flipper will pay for the property. If the rental numbers work out, as you suggest they do, you might also get a little more from a buy-and-holder who's not counting on a quick profit - but you still won't get more than market price.
Once you've figured out what your potential buyer might pay, then you can back into how much you can afford to pay with a little profit for yourself. If that's lower than what the seller will take, no deal.
Good luck!
Michael