We've seen old old sewer lines (1940-1970s concrete lines) collapsed, grown through with tree roots, and otherwise deteriorated and we've seen 2 year old lines with "bellies" that cause them to "skim over" with TP and then back up into the house. 

Well, the contractor was simply doing what was required for you.....even though he should have made it clear the tree roots might be damaged or at least he/you have the roots trimmed/treated professionally 

In either case the root cause is often the lack of surface water management (gutters&downspouts, driveway pitch) and you always want to make sure you not only treat the symptoms, but eliminate the root cause.The other issues with brick foundations is that they are somewhat senstive to water and start to crumble on the inside as water wicks through leaving a crust ov minerals on the inside where it evaporates.

If the floors are damp (which I doubt) you have to find out why and then stop the root cause.

I owned an older property, and tree root issues over time caused backups to the sewer line to the city sewer system, and I had to hire Roto Rooter to come with special equipment, camera to check out sewer lines.

You've got one more fan here rooting for you to get to 200+ doors. 

If you are going to use a property management company, I think it wouldn't make as big of a difference in where you invest.However you will need to build a network where ever you end up investing, and it usually easier to do that where you already have roots.

But where is the root cause of this problem? 

My wife and I along with our two children are pretty rooted in our current community and love it there, but are very interested in REI. 

I always rooted against him growing up.When I dug into his story I realized he has a lot in common with Michael Jordan.