I don't know how much space there is under the hood, but prelubers would be a good thing to invest in. Since we're talking new cars here, it would be a good time to install one of these. From
http://www.carbibles.com I got two sites
http://www.autoenginelube.com and preluber.com, where you could get kits to install a system which gets your engine lubricated before you crank the engine. This will save you from a lot of wear and tear on your engine, since the startup really eats away at metal from the lack of lubrication during that time.
Another time would be to buy magnetic traps that you wrap around the oil filter and under the transmission. This keeps tiny metal filings from getting back into the engine oil and erode your engine parts.
Read
http://www.carbibles.com/engineoil_bible.html about the magnets.
Speaking of preserving your drivetrain, I'll also want to eventually use synthetic oil, after the initial break-in phase, and also, I'd want to use a synthetic-based transmission oil.
As for accessories, you got me into a lot of ideas, and it'll probably end up about a couple of grand, but considering that I'm not spending that couple of grand going with the SL, it will be sweet (Of course, I am losing out on the cruise control, which isn't an option on the S trim and is standard on the SL):tint, mags, rearview dimming mirror with map lights, weathertech mats, seat covers, change to sound system with aux input + cellphone bluetooth. Of course, some of these may not be available yet. Then later on, fog lights, K&N air filters, and side mouldings. If I find a nice design, I'll put on some decals that's a combination of foreground yellow and border black that will stand well against the metallic gray background.