Buying parents house and building new house on our property

We have 30 acres in rural Indiana where we live.

We’ve offered to allow my wife’s elderly parents to put a new modular home on our property so that they will be closer to their family and not have to maintain their existing property.

They owe $20,000 on their current mortgage.

They haven’t been able to sell (asking $110,000, PVA $104,100).

The house is in a good area to rent but probably needs $4000 maintenance to get into rental condition.

My father-in-law is in poor health and will probably only live a couple years. My mother-in-law could very well live 5 or more years.

I’ve offered to buy their house and get them in the new modular ($70,000).

Should they keep their old house an rent it or should I buy it and rent it?
Should they buy the new house (it will be on our property).
Should I buy the new house and rent it to them.
Could they offer me seller financing on their old house? And pay me rent on the new house?

I see that there’s many options and I’m looking for advice on the best options.

Thanks for any advice