Agreed that the buyer is either not knowledgeable, or doesn't want to tell you what they know. He also sounds desperate. Whenever someone is pushy like this, it sends up red flags and makes me want to dig even deeper.
1) Make sure it's legally his to sell.
2) Determine the engine and tranny. Since it's an 06', it very likely has a Allison 2000. Knowing what you have allows you to do up front research before taking a step to having it checked out.
3) Get all the "proof" of it being rebuilt.
4) Have it thoroughly inspected. If you were looking at paying $8k, and he immediately dropped it to $6k, you A) need to know why B) are saving $2k, so a few hundred for a thorough inspection is cheap insurance.
5) Drive it, city, freeway, up hills, make it shift up and down. If you don't know how to do this, get someone who can and go with them.
6) All the conversion work is also under question. Who knows who did it and how safe and well it was done.