No concrete advice how to do this due diligence?  

So when the time comes, my wife will take over on financing and hopefully get past the lending limits with more ease since she won't be tied to the properties in any concrete way. 

That being said we were never able to get down to a concrete number because a lot of it had to do with the additional income coming in.

Are you breaking ground in summer where you won't have any issues getting concrete in or working around spring rains?  

I would suggest doing a little market research in your area and try to get some concrete numbers on what the current rental market is for the home maybe calling a rental agent in the area to get an estimate, also getting some bids on the repairs that need to be done maybe it could more or less for the repairs.

Lastly, I would strongly go by credit score unless there is a concrete story to tell. 

From landscaping and laying concrete (all by myself, I might add) to getting people in there.

Although the bones are in good condition, there is the potential of expensive cosmetic updates in the future, like concrete work, etc. not to mention more frequent minor repairs than newer homes require and potential of future major expenses like sewer lines, foundation, etc.

I mainly need someone who can inspect the outside structural integrity (walkways, elevated porches, concrete, etc.) and the common mechanicals (i.e. boilers).

Maybe I'd be more on the lower end as it's just a single car garage and the concrete driveway looks bad.