I was sued once because a tenant invited me (in writing, via text) to her workplace (citing transportation issues) to pick up a rent check.

It's an efficient way to passively learn while doing other things. 

Market research where tell you where you want to be and where you don't, and give you a good idea of what to expect out of a certain area or submarket, this will allow you to move quickly and surely when an opportunity presents itself and help you with valuations.Analyze everything, start looking at the properties you pass everyday, pick one and research it from address to strike price do this until you are able to get it done quickly and efficiently don't wait until there is a deal in front of you to try and figure out how to analyze it, its to late to figure out how that game is played and what numbers work and don't when the contract is in front of you do that home work before hand when the clock isn't ticking.Finally this, Networking is key both online and in person, look into local REI groups and get active on the forums, get used to talking to people about RE and your goals. 

Then pay 0% tax on those 10 properties.I have been trying to figure out how to obtain REP status to pas losses on, but maybe I want to avoid it so I don’t have to pay self employment tax. it seems that there may be a sweet spot, or efficiency spot, while I transfer to full REP status.

Best thing is to find higher than average income in a B or better area - with an efficient heating envelope and where tenants can cover electric. 

Residential steam boilers are 82% efficient on a good day.

This is not a response to some memo everyone got, or just random chance, it's the best method for operating efficiently, predictably, and with scalability. 

Should I replace with 1 unit (a stronger and more efficient system) and keep the zones for both units?

Looking for some advice how investors with over 100 doors process there bills. We currently have our own PM team in place but each month its a big process to get utilities payed and recorded properly. Looking to make ...

If however the goal is the lowest operational costs possible...I'd say all under one roof is the way to go, to your point, it's way more efficient and standardized that way.