More worried about Mark said regarding contacting the housing authority to ensure I receive the voucher payment on time. 

We have the right to deny him moving in, based on what our lease says, but if we decide to all bunk up what kind of documents should we prepare to ensure we're protected?

We have the right to deny him moving in, based on what our lease says, but if we decide to all bunk up what kind of documents should we prepare to ensure we're protected?

You'd certainly want to re-visit your numbers to ensure that these payment terms you've described here don't jeopardize your financing goals.Personally, I would have proposed to simply make fixed principal payments on the balance over the two year term rather than offer interest right off the bat.

Evictions have been suspended in our area, but can I do anything in the meantime to ensure that I can get the rental payment back in the future and pursue eviction if needed?

I will likely need to set something like that (or a similar maneuver) to ensure I don't miss the next great chance. 

Prior to buying a house analyzing the rental market and ensuring that the true cashflow (rents – ALL expenses) meets your criteria.

It was a huge challenge to get ahold of all of our tenants and email them to ensure safety of their belongings.

I also run initial numbers to make sure it would be cash flow positive and ensure there is some upside potential before I sent it to them.

I agree with @Stephen Keighery, but keep in mind that you may have a hard time finding a good tenant for the unit you plan to rent out, so I would suggest you ensure that you can cover the note without the tenant for some time.