But my brother and I can't afford to invest in rennovations, plus it would be risky since we don't know enough about it, plus I live on the other side of the country.

What would you do if the 88 year old tenant who has occupied the unit for 20 years can't afford a 200 dollar rent increase? 

These areas seem to offer affordable prices for me as an amateur investor but I just want to make sure I'm not wasting valuable time analyzing an area that's largely stagnant.

You may get stuck with a property to can't afford to fix up or handle any adverse situations that arise.

It will also give you a more exact idea of the price point you can afford.  

Is there any general "rule of thumb" for holding on to single-family residence and renting (if you can afford to). 

Strong buyer demand is keeping homebuilders confident, but rising costs of construction materials are weighing on housing affordability.

If you can show the lender that [1] you will be able to cover the mortgage on your SFH (may need a signed lease) and [2] you can afford the mortgage on multifamily property (rental income + your payment), I don't see why a lender would say no to that.

Can you afford one struggling tenant?

i would contact an attorney in this area, it never seizes to amaze me the links the cities will go to so they can cause problems and then tell you they all about affordable housing, all they really about is taxing the heck out of homeowners and they know they cant get lots of taxes out of mobile homes so they dont like them, i dont see you losing this battle cause the homes were already in the park and in lots of cases that grandfathers them in, mayors are not lawyers and it sounds like you are in an area where they dont like parks good luck