Welcome and congratulations! Do you have the terms verbally negotiated already?
There are a lot of things with subto deals that are "dealer's choice" ... By that I mean, there are so many ways to get the deal done, you really need to be apprised of the implications of each fork in the road and make your own informed decisions.
1) Sources of paperwork for the deal: State real estate purchase contract wholesaler contracts, real estate attorney (I feel there's a false sense of security when using most attorneys for creative finance deals), JV partners, and consultants. I'm the latter, and I run 50+ deals per month.
With creative deals like this, it's important to understand your own exit strategy so you get the entry paperwork set correctly. And anyone on the loan must be fully onboard and be willing to sign a long set of disclosures declaring they understand their credit is still tied to the loan that you're now paying (and if you don't pay, it damages their credit).
2) Yes a Power of Attorney will be important, but for so many more reasons than the one you listed. However if you're thinking that's the solution to the due-on-sale, the deedback and CFD re-buy is not applicable in all states or circumstances. There are at least 8 strategies I've been able to count or come up with to handle due on sale. So just be aware that it's not like you can pick the resolution strategy now. You just don't know yet what will be required.
3) Are you talking about a performance deed? If so, beware! It's often promoted as a cure-all safety net for sellers, but it may well be unenforceable as a circumvention of due process of law... Search for a YouTube video called Is a Deed In Lieu of Foreclosure Illegal?
4) Either/or, but of the many attorneys, there aren't many who have a deep understanding of creative finance deals. I guess the same is true of consultants. What you're really looking for is someone who's got lots of reps doing creative deals, whether that's a JV partner, attorney, or a consultant.
5) Missing concerns? That depends on your deal structure / offer, what type of loan it is, whether there's down payment assistance on it, how old it is, if there are arrears to pay back, if it's having/had a loan modification done, whether the borrower on the mortgage is deceased or divorced or soon-to-be deceased, whether you want to buy in your own name or a trust or an LLC, whether you want to actually have title transfer on public record or not, whether you're ready to decide to pay 2 insurance policies or one, .... and so on. I digress.
If the person you're working with to help you on these deals isn't asking you all these questions and helping you make informed decisions, you're being disserviced.
I'm sure that if you've got verbal buy-in from all parties on this deal, you can have it under contract by end of day. Let me know if I can be of assistance.