We're going through the same process right now and are inclined to create one LLC per property with a Corp as controlling entity.Even though some states do not require you to register the entity on that state (in Ohio is preferably but not mandatory for example), if you would to have any legal issues, you would need to be registered to conduct business in that state.

If someone has been taking care of it and try to claim it you will have to deal with the legal process of redemption or try to foreclose on it (this all depends on NY state law regarding tax liens)

Some of the people you dupe into believing that you're going to buy their house and then screw over by reneging on the contract are going to have friends who do unreasonable things like find out where you live and hide out in the bushes near your car with lengths of steel pipe and degenerate pygophiliac designs.

Secondly, when the legal owner dies, who then takes title?

I can't tell you how many non-legal leases I've seen over the years. 

@Carlos PenaYes, either buyer or seller can "change mind" after signing contract.However, contracts are legally binding so there are consequences if both parties don't agree to the change, usually financial penalties.Check with your local real estate attorney to know what the consequences are with your particular contract and whether it is worth your time and expense to enforce the contract as signed.

I ended up doing plans myself for a renovation and all was fine -- but I had an ally at the building department and some experience in design/construction so that helped a bit. 

@Alan FaitelFor legal advice, you'll need an attorney.

So you're de facto realtor practicing real estate without license......do you think it's perfectly legal?

Do not rely on it for legal advice.