Congrats on your solving the automation puzzle.

I feel like I have a lot of pieces to the puzzle down, but I'm missing a few corner pieces to feel confident about investing. 

These types of fixes, generally go fine, you do have to replace a lot of wood, and it can be done like a puzzle. 

The last piece of the puzzle to make well informed offers is to understand the cost of construction and rehabs in different markets.

This is one of the more important pieces to the puzzle that gets looked over way too often.Hope you find value in this message!

That one thing is a crucial missing piece that needs to be found to solve the puzzle of success.

REI feels like a big fun (albiet risky) puzzle to me, and the more I can learn, the more angles I have when approaching or exiting deals. 

Experienced developers are likely going to have all their puzzle pieces in place so money and deal flow would be your prime target for adding value to them in my opinion.

Is there a piece to this puzzle I'm missing?

You're looking at putting 290K of debt on a house worth only 180-190K which just won't work.Either you're leaving out an important piece of the puzzle or this plan doesn't make any sense.