I'm totally good with some sweat equity but I'm not in a position to do Roof, siding, structural repairs and such.

Start by talking to a cross-border CPA and attorney to handle taxes and legal structures properly.

Question: why would one choose to build an ADU when existing structures sell for less than the cost of development?

This will not qualify for 1031 Exchange treatment under the present structure and "use." 

If seller financing is the route you go, you might consider getting creative with the structure to make it work for both parties.

It's a little different but structured properly it beats the work, hassles and uncertainties of flips.

I have NOT placed much research into it but from what I have read their fee structure is expensive. 

My understanding is that the ballpark standard of "unless it would be outrageously tough" (something along those lines) is what applies if you're doing a rehab, seeking new permits, doing structural changes anyways, and things of that nature.For existing pre-ADA structures, my understanding is that the standard is more along the lines of "if it can be easily/cheaply done, you have to do it."

Are there any tips you might have on sourcing great deals, deal structure, and how to know whether LTR, MTR, or STR is the best route to take?

While adding compliance costs (filings, payroll), this structure is beneficial, especially for larger portfolios.This post does not create a CPA-Client relationship.