There isn’t any good reason to not put AS IS on the bill of sale. I just think that the laws regarding a private sale of a used vehicle are what they are, and the idea that an ‘as is’ makes a huge (or any) difference is an urban legend. I could be wrong.
The NY DMV is picky about turning in your plates (in NY the plates are linked to the person not the vehicle). Technically, they can fine you something like $3/day for everyday you don’t, but I’ve never had it happen.
As far as liens go, in NY state any and all liens will be printed right on the front of the title itself (usually its the name of the bank that made the loan), so if there aren’t any there you’re good to go.
Also, I would recommend you insist on getting cash only. Don’t even consider a personal check and even a cashiers check seems pointless to me. They have to get that at their bank so there’s no reason they can’t just get cash instead.