Since the other lien exists it will need to be resolved or released.

A friend approached me to help him with a property he inherited from his mom passing where a developer wanted to buy from him for commercial development.The developer after me catching everything in the contract to tie up the property with the seller asked me to work with him on the other parcels as he would rather be working with me than against me assembling the properties.Off I went after that for about 3 years in land development and then went into existing investment properties.I did things kamikaze style and figured them out on my own through the school of hard knocks.In your situation if you are focusing on apartments I would do the following.

His existing tenant was moving out and he was going thru a divorce.

I'd bet if you called RE attorneys and told them you'd be interested in using them as you are looking for investment properties they might review your existing contract for zip, less than 200 bucks I'd think, ask them to look at it as to "form and content".

I also wonder about the tax impact (full recognition of capital gains at time of "sale") if this is done at scale where the seller may be construed as a "dealer".In my area, quite a few landlords seem to be converting their existing rentals to land contract deals, to get out of the landlording liability and transfer the maint to the buyer, while still maintaining the ability to evict (rather than foreclose) within the first 5 years.

existing financing at the end of the term.

I found some fixer uppers that were sold off market that I could get the seller to seller financing with a bit down and a short term loan (even via wrap when a lien existed).

If the old note still exists, even if it just on one of the properties, the new one would typically be second in line to the old one.

How much do you like managing the existing rental property?

If one doesn't exist, you should sign new ones with the tenants.