Hey Andrew, I'm a land broker who sells land to both boutique builders and to the national merchant homebuilders. 

I think I have calculated the appropriate numbers, but this our first new-build adventure and I would appreciate more experienced eyes on this analysis.We are considering purchasing land in Packwood, WA and building an upscale modern 3/2 1500 sq ft A-Frame cabin.

IE rent to a tenant for >12 mo,  dating before marriage, no late payments then with a down paymetn (long story here)  I'll sell on either a contract for deed (typically mobile homes on land) or a note and mortgage (regular nicer houses). 

Land Loan with 20% down etc.

There is some sort of u-shaped or curved situation somewhere but I can't recall where

Your home needs to be in good shape (sounds like it is) and very competitively priced in order to have the few looking choose it instead of another option. 

Another example of this is Newark, extremely strict laws making land lording a second job.When finding the right area to invest, towns with more lenient land lording laws and proximity to public transportation tend to be a receipt for success.  

I’m going to take the $2,000 for the land out since you will still own the land.  

Depreciate 75% of the building value (excluding land) over 27.5 years for significant tax savings.

Yes the homes are not in great shape... to put it mildly...