As far as I can see the choice is a no-brainer. Sell the 1987 745 GLE Turbodiesel and use the money on the 1988 745 Turbo. Then hook your SIL with the '91. That will give her the auto she wants and leave you with what I imagine would be the better long-term choice of the remaining two... lucky for you they're both m46 (I won't drive an auto vehicle).

I don't care what people say or what arguments they present, diesels suck. Some newer low-sulfur diesel are out now that are better, but by and large my rule is that they are to be avoided unless you are talking about a Semi that needs it. Why would you want a much louder car (diesels are a racket) with less performance capability that is literally spewing out the most noxious chemicals imaginable and smells like a rotting Irish bog. It'll burn on anything... yeah, great, I live in a country where I have ready access to cheap gasoline, so why the hell would I need a car that can run on kerosene.

Besides, if the list you provided is accurate you've almost got the '88 in top shape. You're sure the rods and valves are all in top order? Cat and exhaust system? The headgasket will be the biggest thing you've got left, but you can probably do all the rest (parts and labor if you don't DIY it) for under $1200. Well under what you can probably get for the turbodiesel assuming its in good shape.

-rt