I suspect that is Texas, but don't honestly know so do your own due diligence. 

I've checked out the usual suspects- HD, Lowes, IKEA- and have also gotten a handful of quotes from smaller shops.

We suspect that the neighbors are in violation of zoning laws, such as the RFA (which has guidelines for single family residences) and maximum height laws, and perhaps certain regulations regarding a covered garage.I and others have filed official complaints with the LADBS, but have not had any luck with the city inspectors for our district, who seem to care very little and have done nothing to help.Is there any advice you can give us regarding our rights and perhaps legal action we can take to protect our privacy (the tower is at a height that can see into my home) and our quality of life?

I suspect that cap expense will be at least $200/month (I use a much higher number where I invest).  

I suspect you will  find most private lenders and hard lenders are looking for a bare minimum of 8% but keep trying.

suspect non buildable or cost to build reflects asking price

Only do this if a complaint has been reported or you have actual reason/cause to suspect a lease violation.  

I suspect that there would have then been also things on the inside that were changed.

I'd be a little suspect of your "buyers".

You can record a memorandum of contract at your register of deeds if you suspect the seller may still look for buyers even though they are legally obligated to sell it to you.