I imagine that the length increases by some amount, but I doubt it would be a full year since many tenants feel fine riding out a month-to-month lease until they decide to leave.Do the downsides (forfeit of owner ability to increase rents after the year and potential to get stuck with a trouble tenant constantly calling for minor repairs) compensate for whatever this additional length of stay is?

Where to invest is a more difficult choice and a lot depends on your goals, your investing style and your strengths.  

The newer ones are hungry but inexperienced.When you deal with someone new, (depending on your style), micro-manage the process and follow up/follow up/follow up.  

Additionally, you'll also need to confirm the rents.  

There is work that needs done on the property, so we'd have to be creative with where we pulled additional money from.

I know that's the case around here, additionally rental demand for SFDs are higher than multis which can result in better tenant pools.

Stairs are additional.

Also, had a wholesaler tell me when I first started that he would buy a home and sell it to just make an additional 5-7k on it.

.-  Basement is unfinished but has some existing plumbing, garages, finished walls,  etc. and I conservatively estimate $30,000 to build out each of 2 additional  units

In addition, it could be that the seller has died.