A tentative thought might be to ask folks who are estimating rehab using our calculators if they'd like to share and in return access other people's estimates for rehab projects in their area so that all can improve their ability to accurately estimate costs. 

If everything you say is accurate, I'm not sure why the seller would sell at a discount.

Be certain of your numbers - make sure all your numbers are as accurate as possible.

There are many qualified accountants on this message board who specialize in real estate taxation that may be able to serve you.Since you previously done your taxes using HR Block/Turbo Tax - the accountant you choose should first look at the previously filed returns to make sure they were done accurately. 

I tell my clients on the first day we meet that wire instructions will only come from the title/closing agent as encrypted documents and that they will need to look-up the phone number for the title company before calling to confirm the instructions were sent and are accurate. 

What steps should I take to submit an accurate offer? 

Alternatively, If your numbers are accurate, there is definitely enough spread to work with a HML if you are employed, have decent credit and can put some skin in the game (ie 10% of the purchase or so).

They cost more- but are more accurate.

Developers might be able to make money on duplexes, but people just don't want them that much, the market reflects this accurately. none of this is to say you can't make money on duplexes in the area. 

My only concern with off market is properly figuring the right price to negotiate and finding the ARV quickly and accurately.