I have a 1987 300D and my brother has a 1992 2.5 300D. So I've driven them both and done some maintenance on both.

They are closely related cars, as you know. There are some tradeoffs to consider of course.

1987 has more power, enough to clearly notice.
1992 will get better fuel mileage.

1987 is set up with a firm suspension.
1992 is set up with more of a "comfort" suspension.
This apparently is mostly due to different shocks/struts.

The 1987 cars will sometimes act "cold-natured" for the first couple of minutes after a cold start. They'll put out some unburned diesel smoke, idle rough. Quickly clears up and smooths out. This happens because the glow plugs turn off when the key is released from the "start" position. Nothing to worry about, but it really freaks out people not used to diesels. I think this is why I got a good deal on the car - the owner probably thought it was oil smoke and the car was about to blow up.
The 1992 cars have a different glow plug and relay that stays on for about a minute after the car has started - this solves the cold-natured issue.

1992 cars have wood trim on the doors.

Original 1987 had R-12 Freon A/C - mine was overhauled and converted to R-134a. The A/C may be a weak point on all the 300-series cars in this date range (say, 1986-1989 to guess).
1992 would probably have R-134a, I can't remember for sure.

I certainly like the power of my 1987 over the 1992. But either car is acceptable to me. If presented with a purchase choice, I'd simply choose the better car (better deal). I don't know of any reason to pay a premium for a 1992, except that it is a few years newer.

There is a potential issue with the early diesel 6-cylinder engines that went into the 1986-87 Mercedes - that of aluminum head cracks. Rather expensive to repair. I don't think that has ever been an issue with the 2.5-liter cars.

I got my 1987 300D for $2500, with 325K miles.
My brother's 1992 cost about $7200, with 200K.
His is in nicer cosmetic shape. Mine is pretty good, but has more interstate rock chips in the paint.

Hope this helps.

Ken300D