Naturally, mid-sized more affordable, still charming cities like Richmond show up on our list for that reason.Our extended family also seems to be migrating southward, so there is the gravity that comes with that. 

That's like breaking the laws of gravity!!!...

But if it's a true septic system with a leach field, gravity takes care of it for you at no cost!

Heating systems might be gravity based (not forced air or no circulator if a boiler).

They are a simple gravity system and not that expensive in the whole scheme of things investing.

I like the list you got from @J Scott (and follow up by @Will Barnard ).Adding to JScott's very good advice about getting a copy of any perc test results (if the property is NOT on public water and sewer) is to take notice of whether the lot is perc'd for a conventional, gravity system or an alternative septic system.I don't know about Maryland, but here in Virginia many counties are strongly discouraging alternative systems by adding costs, red tape, regulations etc. when using them.

There are backwater valve for gravity systems fairly inexpensive from the hardware store, however, it can get expensive. 

Use gravity flow for your drain line and you won't have any issues.

Stay away from knob-and-tube wiring, galvanized water supply pipes, ancient gravity-fed hydronics converted coal-burning boiler arrangements, foundation issues.Why a duplex and not a more profitable triplex or quadplex?

What this means is the basement floor is lower than the City sewer main, so cannot drain by gravity.