Does it need work to prevent deterioration like a roof?

I find that as my network grows, the quality of the network deteriorates.

However, residents in the northeastern part of the U.S. have experienced problems with certain residential concrete foundations that contain pyrrhotite, which can cause concrete to deteriorate prematurely.In 2017, IRS provided a safe harbor that allowed taxpayers to treat the costs of repairing such damage as a casualty loss in the personal tax return. 

Are they notorious for falling apart and deteriorating once they reach a certain age?

That of course assumes a few things like, the rent is covering expenses, the neighborhood isn't rapidly deteriorating, basically you believe things aren't in permanent decline.

Original owner was getting sick and let the property deteriorate)...Ideally I would like to open the walls, too and dry-wall from scratch - there is the old house smell and most importantly I would like to make sure whats behind the walls looks kosher.And I would also need to fill the in-ground pool cause its deteriorated to the point of no return.Any ideas as to how quickly rough-estimate this rehab?

It was medical prior.The town itself maybe deteriorating but hospital is largest employer.If the tenants were to stay while he figures that out it would be a good thing.If he uses the space he may only need the first floor at first.The hospital will pay rent bc he is employed by them.I know from MLS the larger business is listed for sale ;(Makeup is florist, antique shop, lawyer, realtor(owner) and bridal (for sale?)

Bad neighborhoods require 3x the due diligence, on vacancy rates, expected turnover, crime/sex offender rates, gang activity, potential deterioration in surrounding properties, possible exit strategies, on and on.

Long since paid off, 'hood had deteriorated, grown kids didn't want it, she wanted cash only, wanted a ridiculously high price, claimed she collected the rents in person every month and everyone paid, no repairs needed....you get it.

So, files deteriorated which caused ownership defects in the mortgages.