should I use a real estate agent to buy an off market property?

[James Richter]

I am looking to buy a unique property. It is not on market right now. Sellers father has passed and the seller wants to sell the property the father has left. It's unique in the sense that it is in what the city zones as "medium density" residential but the property does not have a house on the property. Just a large garage sits on the lot. I believe I can fill it with cars to store over the winter season which should cover the note, insurance, taxes, etc. And if eventually I could build a multifamily home on the lot then that may make a great opportunity to enter the multifamily world.

A few questions I have are:

1). If the seller doesn't have an agent should I bother using an agent? Or should I just have an attorney write up a contract? And if so do I start with a letter of intent? 

2). We are quite far apart on what we value the property at. I am wondering if I should purpose a seller financed deal structure? I doubt the seller needs the money in full right now so maybe it could be the right option for me to pay what I feel is fair and the seller will realize more of the money if we can help avoid the capital gains? How would be a good way to explain how seller financing works? Maybe a short and to the point print out to send the seller home with to review?  

3). I also wonder about getting the property appraised to try to get a better idea of what a unique property like that is really worth. Appraisal × .8 = my idea of a reasonably safe offer. 

Of course I have many more questions but these are the for-most on my mind.  Basically what is the wisest way to approach this deal. 

Any help would be much appreciated. Thanks for reading.