After seeing the pics, I think
@Macrobb 's assessment at around $3K is about the right value for that truck. It does has some pros that put it on the higher end of a typical IDI truck, but there are some cons too. Like whatever is staining the underside of the hood.... some red flags. Also, while the outside looks ****, that engine definitely seen some work in those quarter million miles. Not necessarily a bad thing, if the PO has records showing he maintained everything. But the spray on the hood makes me question his attention. Also, the parts looks old, when you buy this, be prepared for a line-item check of all the accessories. Don't be surprised if you have to start replacing hoses, lines, the vac pump, other pumps, etc.
Ask the PO if he uses SCA's and what type of Glowplugs he uses, and if he doesn't know what you are talking about, then I'd start to get worried.
but the chance to trade straight up is appealing to me.
I'm hoping you are not trading your only car for this truck. You will need a backup vehicle just in case your IDI repairs get extensive.