The seller (me) gets top dollar for a sale, and monthly payments over the term of the contract (roughly 20 years) -- assuming the buyer is properly vetted and pays in a timely fashion. 

.$3750 lot rental$2125 mortgage payment receipts$5875 gross rev/mo.A rough estimate of an undeveloped acre of land in the immediate area is about $5K per acre.No expense information yet, so obviously that is in my list of questions/due diligence.I know his taxes are <$800 year.Gravel roadsNew DEQ approved septic systemCounty services (not the nearby town)What are the questions I need to be asking at this initial meeting?

I have bought from fatigued landlords and from estates and each time the properties were in pretty rough shape.  

Quite frankly, it's tough to manage and/or sell properties in rough areas, especially for newer investors.

That will give you a rough idea of what you could reasonably expect the ARV to fetch. 

However, we will be taking a HELOC of about $55K to assist with the 20% down on the new place.If we sell and net ~ $145-150K, we could use that and our savings (about $30K) to put the 20% down and have roughly $90-95K to begin looking for another rental (or 2?).  

Last night was the initial walk through and I am just trying to get some rough numbers together. 

They likely will want to go back on the market to look for an owner occupant who is willing to do some work.Depending on how eager they are to sell, they may be willing to get renovation estimates themselves - I would double check the sources, but it should give you a rough ballpark of renovation costs, so you can follow up with an official offer.

He will be confirming purchase of a plot of land in two weeks and it will take him roughly 6 months to build the house on his own.So should I, keep my low paying and very secure job with the knowledge that there will always be a full time job for me at the company or  should I ask if I can help my Dad build the house to find myself without a job in 6 months time?

I try to stay with the numbers that HUD has in place as affordable for Section 8 even though a unit isn't getting a Section 8 tenant - 30% of income toward rent (income roughly 3x rent), with no more than 40% of income toward rent (income at least 2.5x rent).