These are just general answers since I'm not terribly familiar with any of the body styles other than W123 like my and your 240Ds.
#1. You can probably adjust the steering box. Like BigPimpin mentioned, it's a relatively commonly discussed topic. Also, changing power steering fluid and filter is a pretty easy task and should be done if you don't know when it was last done, especially if steering makes any noise or doesn't feel smooth.
#2. Have you checked pad wear? Who did the last brake job? Are they real Mercedes pads or those awful "lifetime" pads? I've had bad experiences with so-called "lifetime" pads on my W123 240D. If you're extremely concerned about the noise, you could take the brake pads off, clean the calipers and pads thoroughly with brake cleaner, apply brake quiet to the backs of the pads, and reinstall them. Or, you could wait until you need new pads then make sure everything is clean and the new pads have Brake Quiet on them before installing them. Make sure to get Mercedes pads.
#3. I've never used Kleen Wheels, but others say they work as advertised. Your only concern might be excess heat due to decreased ventilation at the brakes.
#4. Have the new seal and some Weatherstrip Adhesive (3M or Mercedes) on hand. Rip out the old seal, scraping as much out of the seal seating track as possible with a *plastic* (not metal) scraper. Clean the track thoroughly with 3M adhesive remover. Apply weatherstrip compound to the track and/or new seal as directed on the package. Carefully place new seal. (this works for my W123 240D, might be different for yours)
#5. Hmm, perhaps check the "Wheels & Tires" forum or your owner's manual. If the tires have directional tread (look carefully on the sidewall for an arrow) then they may ONLY be rotated front to back. If you rotate them diagonally then you'll reverse the tread, which could have unpredictable results ranging from erratic handling to tires self-destructing. Or, your car might operate normally but when you drive with the tires on backwards, the earth begins to rotate backwards like in that Superman movie where he reverses time to save Lois Lane... anyway, you get the idea. Check for directional tread before rotating.
#6. I removed and cleaned my dome light which was doing the same thing. It was hard to disassemble and reassemble - took about 2 hours for the whole job - but has worked fine ever since.
#7. You might look for Hella replacement bulbs, which legitimately improve your lighting but are somewhat expensive. Please avoid those tinted blue bulbs; they're ugly, ricey, and illegal in some areas sine they don't provide as much illumination as normal bulbs.
#8. The pushbutton A/C controls in the later W123 body styles are notoriously finicky (i.e. they break). You may or may not have the same pushbutton system. I luckily have the manual climate controls.. sorry, I can't help with this one.
#9. Check the "Detailing" forum for leather cleaning. Also take note of David C Klasse's problem with Lexol:
I have MBTex seats with sheepskin covers. It occasionally drips in the same way but I'm not terribly concerned about it.
#10. Sorry, don't know. Springs don't last forever and can begin to sag after many years. However, springs aren't an easy DIY changeout job: a compressed spring of this nature holds a LOT of energy and can potentially maim or kill a person if handled improperly. Your best choices are to ignore it (like many of us do), have a pro check it out if you're really concerned about it, or just tell all your friends that it's lowered. Slammin!!
'83 240D "Steiner"
'00 New Beetile TDI