The stairs up to the second unit may need to be replaced or reinforced.

I had the carpenter reinforce the corners and build a frame under the sink base so it wouldn't collapse when tenants put stuff in it.I installed RTA in my own kitchen several years ago.  

You see a lot of small cracks, crumbling of concrete, exposed reinforcement (if any due to the age of the house), and a lot more cracks due to integrity issues.   

., incentivize by saying the purchase price will be $285,000 if they exercise their option on or before 2 years, or negative reinforcement by having set rental increases if they do not exercise the option by x date?

I will say, it could be worth considering a shot at an actual software solution built for DIY landlords (1 - 15 units) that helps you to automate your own rental process (ie. compared to other tools, one that helps you to actually build a single listing and syndicates it to 10+ major listing sites, allows you to schedule showings, provides you with super thorough tenant screening options, online digital leases which can be customized and help you by breaking out those fees, automated maintenance tracking, and fast rent collection which also offers your tenants the ability to report their positive on-time payments to the major credit bureaus; just another positive reinforcement/added incentive to get them to make those payments on time).

We have tenants lined up once the construction on reinforcing the building is complete.

Use words that keep it a together thing, it's you and the tenants working together, DONT use me, I, you statements which reinforce division and an adversarial relationship.

This will clarify and reinforce what you are reading.

Your husband may be in the best position to guide the tenant back to a more appropriate way of paying his rent and can help re-establish your role as property manager by reinforcing what it is you do. 

Celebrating milestones boosts motivation and reinforces positive momentum.