Assuming the rent from the rentals is covering all costs and debt repayment, hopefully with some positive cash flow, it would be counter investing to pay either of them down.

The plan is to get a home equity loan to cover the renovations (possibly convert to a duplex). 

I do know he owes taxes on the property but would be covered by the purchase price.Is there anyone here that can explain this process and how it should be done?

My son, since he's graduated might pay a low amount of rent, but we would cover my daughter's rent until she graduates.

It’s a Normal Sale as mentioned....sign a contract, get a title search and title insurance, go to closing, the taxes get paid out of the seller’s proceeds....assuming the purchase price covers all the outstanding liens. 

FHA has a "self sustainability" guideline on 3 and 4 unit properties -- this means the property itself needs to generate enough income from of the other 2 or 3 units (other than the one you're living in) to cover the carrying-costs of the property 100% (mtg, taxes, insurance, PMI). 

If you're going the umbrella insurance route, make sure it will cover you for several things including just the routine slip and fall (like mold or earthquake).

If this is something you regularly encounter with new tenants, I would recommend getting a checklist lined up for lease signing walkthrough so you can make sure all your points are covered before the keys are even handed over.   

Second, you need to cover a wide territory with minimum time so don't waste too much time on unmotivated sellers.

The criteria selection is extensive however depending on your County I’ve found that some of the criteria isn’t covered in the records so when searching it fails to result any.