Start low and see what happens.

They're not too big, one is just over 5,000 sq. ft and the other is 13,125 square feet, but I got them at a low price... 

In the market for a good, preferably low cost plumber in the Dayton/Newport area- One of our rentals is not getting hot water, thought I'd ask here to see if there are any recommendations from people in that area.

Generally for Ohio, except in better areas of Columbus, this cap rate is too low.   

Do they want to help me analyze ARV or run countless comps just so I can squeeze out as low a purchase price as possible?

If they specifically need to cash-out to buy another home than seller financing is often DOA.As mentioned, if the have a balance on their underlying mortage- If they want to pay it off it might require a big downpayment- It is possible to leave it in place with a Wrap but that is an advanced strategy and while fairly low risk, the due on sales clause is a concern.On interest rate, my experience is that a seller finance buyer usually has to pay a few points above the going interest rate for conventional/FHA lending so probably closer to 7% these days although it never hurts to ask for lower...All of this is theoretical so the advice to chat with seller to get more information is the right next step.Jeff

Adding bedrooms or bathrooms, low price per SQFT, etc.    

That's the reason I have money, but my credit score is low. 520 as of this week.

Normally no more than financing (hard money lenders will require an appraisal) and inspection.I maintain that the worst thing to happen to flippers is the "HGTV crowd". 

Those types of buyers even if you do find the property tend to be inexperienced so you will be using up all of your time on one deal for a low payout.