The advice given here is all pretty good.
I don't see how hard your dad works or how many people in your family affect the advice you need. Either the car gets the proper care and lasts a long time, or it doesn't, simple as that. We ALL like to think we work hard for what we have, unless you inherited a fortune or something I suppose.
In regards to skipping something that seems trivial, like say the climate control filters, y'know, go ahead, skip it. It'll eventually bite you in the a$$ skipping things though, for example the air flow will smell stinky and eventually restrict air flow, make it hard for the blower to run, then you need a filter AND a blower and who knows, a blower regulator. Cha-Ching! NOT worth it. The car has climate control filters, and you guys bought the car, so deal with it. Just like for example someone buying a house with a pool (no, not me, this is way out of my league, don't get the wrong impression) and then never takes care of it so it's all full of twigs, slime and leaves. What sense does THAT make, y'know?
Your 99 is the V6 motor, one piece of advice I have is to use synthetic oil, as MB currently recommends. You'll also see lots of advice concerning the transmission fluid on this model, my personal recommendation is to just leave the transmission fluid alone. Don't even check it, if it ain't leaking it ain't low. Try to do tire rotations fairly regularly.
I'd recommend also using the FSS system along with the synthetic oil. If you aren't comfortable with going that long without an oil change, I'd at least go 7500 miles between changes, the engine holds 8.5 quarts fer cripes sake, not 4 or 5.
Just so you have a rough idea of the service intervalc on some of this stuff, first, the transmission and diff fluid has NO service interval, leave it alone. The basic oil change is done using the FSS indicator in the dash, you can reset it yourself, it tells you how in the owners manual, and for cripes sake DON'T tell me you don't HAVE one. GET one if you don't, call your dealer or Mercedes 800#. Oh, and don't reset it unless you DO the work. Tire rotations I'd recommend every oil change. The dust filters you mentioned are done every other oil change, they cost around $20 for the pair. The charcoal filter is done every 45,000 miles. If you never use this feature (button on the climate control panel) then I'd go along with your dad and not change it (it costs almost $200 too). It only is active if the button is pushed. If no one in the family has lots of allergies, then I wouldn't sweat that, but the dust filters are always active with the climate control on. OK, the air filter and fuel filter, every 60,000 or 4 years, and the spark plugs on a 99 are done every 100,000 OR 4 years, and youmay be getting close on the 4 year mark on a 99, so watch for that, the production year/month is on the label inside the drivers door jamb, on the car body side, a black label, the month/year is on I believe the upper corner of the label. No wait, it almost HAS to be over 4 years old, a 99 would have been built last half of 98 or first half of 99, and this is July 03, so yeah, count on doing the sparkplugs, all 12 of them, 2 plugs per cylinder. OK, and the coolant is supposed to be replaced every 3 years and the brake fluid, very important to replace it every 2 years.
Make sure you check the air in the spare at least every other service, people let these go flat all the time, what a pain in da a$$ when you not only have a flat, but the spare is basically flat too!
Be aware that the car has roadside assistance on it yet too. Free stuff you can still get are coverage for a flat (they'll come out and put the spare on for free) out of gas (they'll bring you 2 or 3 gallons of fuel free) and free jump starts. USUALLY they'll send a dealer tech out to do it too, not usually like "Barneys Towing Service".