But direct mail isn't cheap, you need to have a good budget and keep mailing. 

we just did our first ones of these on some new builds.. they are energy efficient as heck we got a 100 % score from the outside energy audit company.. they are a little bulky on the walls.. and they are not as cheap as standard duct work systems but .. it does allow taller ceilings within the same vertical envelope so that was cool.. and little quirky to use. but once that's done seems to be OK.. in a rental setting though I would be a little worried each unit has separate controller so if you have 4 ducts your going to have 4 controllers I can see tenants losing them  destroying them etc.. so check that out. 

My thought is, test this method, make sure I pay my lender back (even if it comes out of my pocket) and see how cheap I can buy.

I see ones in my area go for really cheap prices... 50/60/70% of the ARV, just dont know if its free and clear when you bid on them and win or what.. 

Sure you can buy cheap junk software that will keep track like an Excel Spread Sheet or online programs, but when it comes down to protecting yourself with documentations in regards to an eviction, nothing will compare to QuickBooks when it comes down to evidence that you will need in a court of law.  

The worst thing you can do in this situation is try to do it on the cheap. 

Just because something is cheap doesn't make it a good investment, the opposite is also true.

The best tenants were in my condo's that were not as cheap as house apt's. 

I understand the advantages of the technology and processes plus in some areas land being cheap. 

I've done it before with no problem with an attached older home that I got cheap, cheap, cheap.