Double taxation would be when you have to pay US income tax and then also Canada income tax and you don't receive a credit for your US income taxes.

I would give Prime Tracers a shot, they are pay as you go and you also get 100 free credits on sign up to try them out. 

You might want to check with local credit unions or community banks, as they’re often more flexible with second mortgages. 

Before you get too far, I would go talk to every local bank and credit union in the area you want to do this. 

Generally, the properties that benefit the most from a cost segregation study are properties that are remodeled, expanded, purchased or constructed after 1987, the year in which the Investment Tax Credit was enacted.

and how well my credit score will be?

If you develop a great relationship with local credit unions, you'll be surprised by what all they're capable of.   

Your best bet for that size will be local banks and credit unions, good luck.

We have credit scores over 730 and have been "soft approved" for around $750k. 

The seller doesn’t need to know that the underwriters may give you more of a hassle than other buyers.Or do you have lines of credit or a private lender so you can make a “cash” offer then try to get the financing through before close or at worst just do a cash out refi after close?