At first, the functionality seemed too complicated.

Picture 2, ( should be rotated clockwise 90 degree) its a lump above the staircase.

They even mentioned that the owner couldn't figure out how to make the upstairs plumbing fully functional.

Without specifics on your project here I'm going to have to go with some blanket advice -- here's what I do, and would suggest:   use a construction loan from a local bank and they'll fund something like this:- 85% of the purchase price and 100% of the rehab costs up to 75% of the appraised value of your finished product.That will mean that you'll just need to bring 15% of the purchase price of the home (plus closing costs), so that should keep you in the game AND functions nicely when it comes to refinancing because you'll already have a good idea of what 75% LTV is on the property.

As you likely already aware, there are a multitude of ways to make money in real estate and success can sometimes be a function of picking which one to focus on. 

I would flip this question around and evaluate this from your side:  if you view this as your forever home and it truly has that level of sentimental value and functional value to you and your family and staying there for the long haul is a desirable thing, then you'll find that you have an extremely desirable BATNA (best alternative to a negotiated agreement - from the book Getting To Yes) and you'll likely be best served by setting the terms for what you can and will consider in the negotiations with the developer.I primarily recommend you setting these terms (for example: go to the developer and have a frank conversation about the price you'll sell at and let him/her know that this its not a high-ball/low-ball game to you, but you want to communicate the best monetary way for them to achieve this win at a number that is workable to you) because allowing or requiring the developer to set the initial offer amount is going to anchor them on this price. 

We’ve decided to categorize which amenities will be included with rent, but doesn’t make the price too high, but is still functional for us to manage.

We do have some of the functionality you're talking about like agent, lender, etc.

I cant figure out how to do that..maybe if the search function was easy folks would do it more..

I plan to have an inspection done but I’m concerned I won’t be able to get a mortgage for the property if the septic is not functional.