Too close and you may end up paying out of pocket in some instances if unexpected expenses arise

While the unspent money can be used to invest in other properties or use it as a reserve in case something unexpected happens.

You may wish to maximize contribution to a Solo401k first because you can elect to have a Roth component as part of your Solo401k.

As @Christopher B. pointed out there are many components to be considered, and quite frankly many more.That being said, "How do I get an actual cost estimate" and "where would one find an investor, if needed". 

Do you guys put separate systems in each unit or do you put one base station in one unit and connect all security components in and around the building to it?

You will want to have cash reserves or access to cash in the event you have a large unexpected expense come up, such as a roof replacement, HVAC, etc. 

Depreciation is one of the largest expenses to offset income, so it can result in unexpected tax burden.

For newly constructed, purchased or renovated properties and also retroactive generally over the last 10 years, building components are properly classified into individual units of property and accurate recovery periods for computing depreciation deductions.

The "Wall Removal" component woould prompt the requirement for a full building permit compared to a "Pull and Replace" Easy Permit option...

@Bob RandolphI am sorry your investments took an unexpected turn.