Very orginized, and I deal with one of the owners directly on all issues.

Because they're on a fixed income a duplex would be ideal so it could generate some of the funds to cover the mortgage.  

In the situations I've seen the roommates choose the new person who moves in when another one moves out, which isn't ideal in terms of screening, but on the other hand if you force someone on the roommates and they don't like the person that could be a potential issue as well.

Second question, is there an ideal structure for taxation reasons?

(That's how I understood your post, at least) If that's the case, I think you should definitely get your own buyers agent, and one who has worked with investors- ideally, one that owns investment properties themselves!

There is some concern in the legal industry, I deal with hedge funds, that the series LLC concept is still untested in the court system.

Ideally, others could contribute to pages for their neighborhoods, and a homepage would showcase a sampling of what's on the neighborhood pages.I live and some own rentals in the NW area of Philadelphia (in W. 

The trick here is ideally having multiple buyers before offering to purchase - not an easy thing to do when starting an REI business, and even less so in BC, where profit margins are slim.Regards. 

If you qualify for it - truly self-directed Solo 401k would be ideal option for you in my opinion. 

Look at what works best for you and get good advice (ideally by those with both credentials and street cred like rentals or rei background).