They are probably not going to admit over billing and refund us any money.In the future, how do we make sure their bills are reasonable and honest?

Your future in-laws may have the connections to get a construction loan and a  take out loan after the fact but it may be years before a decent, vacant lot becomes available.  

No one can say for sure about a year in the future.

I took my ARV and reduced it by 25% for the future loan and arrived at a 20k budget.  

@Caleb Jordan  No real feedback other than you picked-up a lot of experience on this deal that will carry forward to future deals.Most folks out there would have over-spent on the house purchase, over-spent on repairs and then lost 10-20k on the sale side. 

@Kevin JohnsonPut simply; Pros - They’re experienced and it frees up your time to focus on future deals.

Right now I'm still " house hacking and living i the small unit and renting the larger to suppliment my income and allow me to pay down pricipal fast and save more for future investments.

I'm searching for deals daily and learnig more and more on BP and hope to invest more aggressively in the future but this is all I have for now . 

Those fixtures are in good shape and I was planning on doing that in the future between tenants.

Talking to a lawyer or accountant about the future tenant paperwork?