J.M., I need to start by saying that what I say is neither legal nor tax advice. You should consult a good lawyer or a good cpa for expert advice. I should also mention that the U.S. has tax treaties with various countries and what I am about to say may not apply to you, a Canadian.
You mention that your goal is to buy an REO and then flip it. Do you plan to do just one deal or are you planning to make a business out of it? If I were to do just one deal, I would probably not focus very much on the entity structure and just do it under my name and avoid the costs and complications of a business entity.
But, as is more likely, you are probably planning to do many transactions and not just one. In this case, it is best to have an entity that limits your liability. If you are planning to do all your flips in one state, then your decision is easy - create your entity in that state. If you plan to flip in several states, you may wish to consider a holding entity in Delaware or Nevada with subsidiaries in each state. Or you can register your holding company in each state as a foreign entity, but I do not recommend that approach. One other option, if you plan to do only one transaction in a state, is to consider doing it from your out-of-state entity. Most people on BP do not recommend that option because it can run afoul of the "doing business" laws of the state. But I have done it before and think that a one-off transaction can be claimed not "doing business", although I admit it is risky.
Now, regarding your dad and his money.
You may recall that I had mentioned that interest on a registered portfolio debt is exempt from U.S. taxes. You will encounter two problems with using that exemption. First, that exemption is only available to a loan that has no U.S. connection - the foreigner will have to wire it from his foreign account to the borrower's U.S. account and the interest has to be wired back out of the U.S. As the money has already been wired to the U.S., you guys have screwed things up a bit. Of course, you can undo the whole thing by reversing the flow of money, but you have one more problem to deal with: The exemption is only available to loans from third parties. Your father is considered a "related party" and therefore a loan from him to you is not considered a true loan. (Father-in-law is not related, but father is.)
Regarding the documentation, I always like to have loans documented. This prevents future misunderstandings between the parties. Even if you are unlikely to have a misunderstanding with your dad, others could create problems for you. For example, someone could sue your dad, if he has other assets in the U.S., in the claim that he is the de facto owner of your company. Or some government entity may try to impose withholding taxes on the profits with a similar claim. Or someone may do due diligence on your company and find that it is lacking paperwork on some critical transaction.
Obviously, if your father lends you money to flip houses, he will expect you to pay him interest on the loan. Based on what I have seen, loans to flippers secured by a first position deed of trust are often earning interest of around 18% or so, after accounting for the costs of points. If your dad is giving you an unsecured loan, I would not be surprised if he were to expect a higher rate of interest from you. Obviously, he will want to lend you money from an entity in a tax-free state so that he does not have to pay both federal and state income taxes on the money. Since your dad is a related party, be careful to document the justification for the interest rate on the loan by contacting HML and unsecured lenders in your area and getting rate quotes from them.
These are just some thoughts that come to my mind and I am by no means an expert on these subjects.