The merc sounds like the next level SHO, has it all just does it better. My opinion on the Lexus is that of a similar situation I face with my foxbody. Keep the project knowing it will only be so great or move on and diversify your stable. For me it's about which pony gets fed, I have the truck, the sho, the fox, the postal jeep, and the wife's car. The truck gets fed over the fox simply because I use it, whereas my mustang even with a big block won't be as quick as my taurus so I'm asking myself more and more if I should cut that one loose even though ive had it since 19.
Back to you, if I were in your shoes and had 600/600 at the wheels I don't need my race car anymore, but to have a truck do truck things is very nice. Not sure what your preference of diesel is but for pre def reliability and pulling power I'd go for a 99-03 7.3 Ford and I'll bet your Lexus would net you enough to pick one up no problem. That gen was intercooled and a little beefier than the 94-98s
The 6.0 is a labor of love, same with 6.4 and then the 6.7s had Def I'm pretty sure. Dmax and cummins are high priced and early dmax and post 5.9l cummins had problems
Of course if simplicity is your aim and want it to run off waste oil and moose **** then a good old fashioned IDI will get the job done at 55mph, no more no less but it kills any flying insect in a 500ft radius.