D-17 Purchase

I have not seen or driven either tractor 

I would suggest two two other considerations:

i) For me, a 170 gas or diesel would be more desirable tractor for daily chore/'around-the-farm' use. If you're set on a diesel, the Perkins used in the 170 had a reputation for being pretty much indestructible and only sipping the fuel.

ii) I've done a complete flip on my opinion of gas vs. diesel on old tractors. For a heavy tillage tractor or one running heavier loads all day (like a round baler), I still prefer a diesel. But for smaller chore tractors or tractors only seeing a few hours of use each week and especially for older/antique tractors, I've switched to strongly preferring gas. The last four or five old tractors I've bought have all been gassers, and I'm very glad I went that route instead of diesel. Gassers can usually be had for quite cheap. With the higher cost of diesel these days and most old gassers being surprisingly efficient, gassers don't cost much more to run per hour. If maintained reasonably, they'll still last decades without needing any major overhaul work. And when they do need work, they're much, much cheaper and easier to work on/overhaul. And there's a lot larger selection of decent older gas tractors of that era (including D17's). Very few people are getting rid of diesel tractors until they're completely clapped out. But there are loads of gas tractors (including D17's) available all the time that will run dandy for years with nothing but a carb kit, distributor tune up, and possibly a fuel tank cleaning. All stuff you can do at home for less than $100. Even when they do get worn out, a rebuild on a gasser is usually a pretty cheap, routine and (comparatively) easy affair to do at home. Even if you find a diesel in great shape and it lasts 30% longer, when it does need work you'll have the extra cost of injector/injection-pump service, higher parts cost, more finicky rebuild requirements (valve protrusion & fire-ring/sleeve protrusion) requiring more advanced machine-shop work, etc.

I will concede that the D17 diesel is a pretty cool tractor. But not something I'd want myself, unless I was interested in how unique they are.

What series of d17 are you looking for?   The series four has slightly more modern hydraulics that the earlier ones, with a pump on the side where the belt pulley goes and the hydraulics don't stop if you hit the foot clutch. It also might have a factory 3pt hitch vs an addon one.  The add-on ones are good as well.  Any series of d17 would be good.  What are you planning on doing with it?

On a side note I have both a 175 diesel and a d17iv gas and both have their place and are great machines.