Yep, I know what you mean... Jag designers do a nice job. Even my POS XJ12 was gorgeous to look at. When it ran right (rare), it was wonderful... really like the new XKs and understand they're pretty good reliability wise.
My experience with several of the yards that list on car-part.com has been excellent. If you give them the number from the listing, they will tell you what car and how many miles are on it. They'll also guarantee the part for a limited time. I've found they generally don't answer emails so a phone call is best.
The menu they use on their web site makes it clear that your steering box is used in a number of W140s... good news. As you know, a lot of the Coupe parts aren't common. Ought to be fairly easy to tell on the bench whether or not its got a lot of play in it.
I've got the procedure/service steps for removing and replacing the steering box... if you want me to send it to you, send me an email at firstname.lastname@example.org
. Doesn't look too bad... but, have never done it myself.
There are also procedures for adjusting the friction and "play".