If you go the Jag route, I strongly suggest an X308 XJR. The non-supercharged versions got the typical ZF pile of horse-pucky while the SC versions got a much better (but more expensive to fix) Mercedes transmission. If the ZF goes bad, you have three choices. 1) take it to a rebuilder and pay a hefty price. They will try and fail a dozen times before they start telling you "no that's how its supposed to be" and stop taking your calls. Rebuilding a ZF is like braiding Rapunzel's hair. If it doesn't end up just right, you have 50' of hair to unbraid and try to fix. 2) purchase a factory reman for $4000 or more, or buy a $1400 junkyard unit and say a prayer. The mercedes trans is far beefier. It's expensive to rebuild, but it can be done well by many builders.
All X350s got a newer ZF box regardless of engine, and a full aluminum body with no provisions for any kind of hitch, and (from what I understand) no real meat back there to redneck your own version of a hitch. But I would imagine with 1500 lbs it could be engineered safely.
I wouldn't say an X308 is all that roomy in the back unless you get the Vanden Plas, but in an X308, getting the supercharger AND the long wheelbase meant Super V8, of which they only sent about 3000 stateside during the entire X308 production run. I've been looking for one for three years. Found a few but didn't tick enough boxes to lay down cash.
The 300/Magnum will fit the bill, but I wouldn't call them luxo in that lineup. Comparing a Jag to a Magnum is like comparing a Bentley to a Willys.
I personally steer clear of any E class after a W210. Personal preference given my lack of disposable income for things like repairs. They aren't terrible, but they can contain a significant burden to the wallet. Same could be said about the Audi, but I didn't keep track of them past about 03. They may have improved.
Before deciding on the Caddy, drive one for a while. On paper, they sound great. In driving, I find them to be very not so great. They feel more Caddy DTS from 1996 than a contender with Jag, Merc, or Audi. The electronic controls everything you don't want it to. Shift points? There are no shift points. At least not that you can select with a button, lever, or throttle. It is totally on dial-up. You floor it to pass, it dials a rotary phone number to the ECM, you hear the AOL modem handshake squawks, aaaannnnnd.... then "you've got downshift." God forbid you decide to change your mind on the pass and let up. Right about then, it finally downshifts.... so another phone call on the modem and wait for "you've got upshift." Mom had one. She never passed, nor did she have a clue on what a downshift was. She's a turn key, put in drive, press the pedal kind of driver. Every time dad or I drove it we wanted to roll it off a cliff and take the insurance check.
