Aside from the resale consideration, what will the impact of the location be on maintaining high occupancy with good tenants?

Unless it is tenant caused damage it is your responsibility to maintain your property.

Do you charge for monitoring and maintaining vacant units?

If you own it, aren't you responsible for maintaining it?

Should there be a separate LLC to manage the rentals (lease, collect rents, fix and maintain, etc.)? 

It's an historic neighborhood originally constructed of primarily SFHs right in the center of a metro area in a beautifully maintained neighborhood.

Let the attorney do the dirty work and we can concentrate on getting the deals.

If you were to concentrate on learning everything there is to know it could conceivably take 6 months to a year to consume along with properly researching your market.Learn your market, finances, expenses, cash flow, tennat screening, PM screening, state landlord tenant laws, etc. etc.You are at stage 1 in a 100 stage learning process, slow down.

It has been well-maintained overall, but has not been updated (knob and tube wiring, radiator heat, no central air, plaster walls).

Is there a strong concentration of Marxists in Milwaukee?