@Paul Taing - are you going to get permits and make it all official-like?

[Not official financial advice]. 

Again, this is from someone who has yet to make their first official deal (I was a landlord for a couple years when a condo would not sell, but no longer own that property).

I am wondering what's the best way to document my payments to them since it's likely that they can't provide any official invoice and I will be paying in cash or check.

If you've only done an LOI you need a lawyer to draw up an actual contract and get the terms officially agreed on.

@Ben Ballinger, my area is also 3 days to update the MLS, once the offer has been officially accepted, meaning all parties have signed the same document.Verbal acceptance is not officially accepted, and therefore the MLS doesn't need to be updated at that time. 

Yes even though there are only 39 official cities and towns, there are hundreds of smaller areas which have their own names.In some cases you actually have to use the alternate name in order for mail to get delivered correctly, e.g., Saunderstown vs.

They paid him rent (with no official lease) but everything was in writing from the purchase of the property.I personally would never do such a thing, but it has been done.

One thought, I've been involved in an older office building, a mid sixties building, and the lobby (renovated) and common areas needed to be addressed, at least for budgeting.

We don't have official numbers yet on the additional 5 units so we're just speculating options.