On the legal side of things, make sure that you get material waivers and gc sworn statements from your contractor's to protect yourself from Mechanic's/Material-man's Liens...

I'm a 26yo mechanical engineer working in NYC and living on Long Island.

It all gets stripped to the outer walls, mechanicals replaced, and upgraded anyway.About the only things we consider to add structural value to a "distressed" SFR is the remaining roof life, HVAC upgrade (gas instead of oil, central air instead of window units), high ceilings (of the lack of low ceilings), replacement windows, and actual above grade square footage.  

I hear people say it all the time, but what are the mechanics?

You want to purchase and own the materials yourself, even if you have to go with the contractor/sub to do so, and it may be a pain, but having rightful ownership will at least mitigate any future disputes that may arise.On the same note in CA, and likely in most states, any contractor can put a Mechanics Lien on your property's title pretty much at will.

The potential for overhead leaks increases the more mechanical joints you have.One way to minimize that is to not have any joints at all in the attic. 

They also get substantial discounts on materials and mechanicals and better labor rates from their contractors.

Is there any sort of charging order or other mechanism for creditors to get access to my assets?

Now, I would not expect that any 1/2 full park with lot rents of $100 doesn't have substantial deferred maintenance on the systems and the mechanicals, so my view of price would be a whole lot lower.Do not factor rental homes or their income into your calculations.

Anything mechanical many landlords just take those out all together.When I had my apartments many units had those old disposals in them.