This is a common strategy that I help my clients to set up. 

This is besides for illegal units, very common here, folks collect rent in cash and it don't show up, so if I was a seller doing that, I wouldn't want to spill my guts and say "hey, I collect another $1,000/month that I don't report".

I think it's rude but it is so common that I have moved to "notifications or "opt out" communications.

I don't have the energy to play cops & robbers, that's not my job.Separately, as part of management principles, I would always pay attention to patterns, who's the common element in said pattern. 

Duplex tenants would be responsible for landscaping whereas the fourplex will require you to mow the lawn or clean the stairwell and common areas.I would probably take the duplex just because it would attract families with a little more money and probably have fewer complaints of noisy neighbors, parking, etc.

A negative response in naturally the most common response and for the best - it saves you time.2.

Make sure in the contract it allows you to do this, but yes, this is a very common practice.

Don't be scare of slate roofs, they're common and not an issue, just always have a slate guy go and inspect and repair any broken/missing tiles and flashing on day 1.  

Are rentals common in the area?  

Basically the idea is that if you calculate the depreciation with the common basis mentioned for 27.5 years, you have to depreciate your expense over the next 27.5 years no matter what.