If he puts up the lot and gets the construction loan, what's the best and easiest way to structure this so that we are both 50/50 partners?

It also means you are paying an extra fee for this middle man structure.  

How should I structure a deal like this?

You should definitely weigh vacation rental vs other methods. 

I'd suggest researching the different types of investing methods and then choosing one that interests you the most and mastering it.

I dunno man - it's lazy advice and only hits one method of investing.

I plan to use the BRRR method on this and just found out the house next door is for sale and might be next in line.

Is there a way that we can structure this so that we can purchase a rental and have it be a good investment for both of us? 

NOTE: This does not mean what they think they can get for the property financed by anyone...... it's what they think they can get for the property.... it may require waiting on the right buyer)So each method is different and each has a different purpose.

He did mention that there is no immediate issues near the house that would cause this (like erosion or similar issues) so it’s likely just the natural settling of the structure. if we had gone ahead with the leveling it would be around 18k.