Hmm, that is tough. Didn't they make the 140 with a diesel until 95 as well? They are probably hard to find if they did, plus the whole rod-bender thing (although it may have been fixed by 95?)
Anyway, while I am horribly biased
against Hondas, the generation of Accord you have is by far my favorite, and probably what got them the reputation they have today. This gives you two good cars to have to decide on!
It's tough to say, especially if you are only going to use it on the weekends..if it were me, and I was going to drive it that little, I would not be afraid of going for something that isn't the most reliable; i.e., have a Jag sitting around to go joyriding in. So in that case, I'd say go for the 140. I can't really help you on what to look for though. Obviously I think you should keep the diesel, but it sounds like you know what you want..at least you still want to have a Benz!
I agree with Tyler though, I would much prefer a 90-91 w126 over a w140, but that is just my opinion.