Hi, does anyone know a calculator to use for seller financing which includes amortization?

Yes, check w attny as per @Ben Trageser.Also, see if you can negotiate with the seller to get the illegal unit delivered vacant on closing. 

Are you using hard money loans, tapping into savings, partnering up with others, or maybe using seller financing?

Has anyone had good success wholesaling out of the Dfw area and what tactic worked best for you on finding motivated sellers?

Would sellers be comfortable?

Sellers will also like that better.Finally, don't compare your offer price against the list price.

Do you go direct-to-seller to find your deals or do you simply purchase from the MLS or wholesellers?

I’ve had several investor clients go with a new (to me)/ unproven lender because of a slightly lower rate promise only to get burned by that lender (closing costs were higher, appraisal costs much higher and appraisal was a nightmare, main point of contact gave us the clear to close as we passed our financing contingency then just before closing someone from the underwriting department we previously didn’t know existed said we need a few more days and seller didn’t agree because they had a higher backup offer, jeopardizing the earnest money and losing the property, etc).

It doesn't sound like you visited the market, the PM, or the seller in these markets.

We agreed on seller financing terms, and I acquired a cash-flowing property with minimal out-of-pocket expenses.