To prevent the seller from backing out, selling to someone else, dying, or otherwise not being able or willing to sign to sell when the option is executed, my attorney gets everything signed when the option is started, including the deed.

And get an inspection, and make sure you have any and all contingencies included to cover yourself.I'm not a lawyer and these are just general ideas, not advice.

What language should I use in the new lease to indicate to the renter that I have promised him that I will include his lease in any purchase agreement and advertisements indicating the buyer must honor the lease?

Depending on levels of finish I would throw $125 (min) per sf for new construction not counting demo. 

All three are scheduled to finish within the next couple of weeks (hopefully).

I have done a lot of analysis on MLS listings including the more detailed Frank Gallinelli version of using NOI and cash flow to derive IRR and shortlisted 4 multifamily units from 100s that I looked through that I want to pursue and which offer a good COC and cash flow per unit.

This would include grass, tenant issues, possible handyman work, snow shoveling, etc.

A real estate agent is the best person to help you with this because they have real data on sold properties.Once you know the market (including market rents) and the property looks like a good value, get it under contract.  

It is honestly too hard to tell given I don't know what original site conditions look like, how much dirt has to be moved, are they having to import water to assist in compaction, how much excavated fill will have to be disposed of off site, concrete costs per cy including all fees to deliver, rebar costs, pumping or no pumping, -- looks like you are going to have 2 mobilizations,,,, one for the footing and the second to pour the slab..... there are a lot of variables here..... 

Master bath remodel included new tile for shower enclosure, new mirror and new lighting with LED lamps.