Jason,

Adjusting valves is about 1 to 1.5 hr job for an equipped garage. For a DIY
first time, it would take a bit longer. You need a set of offset wrenches
plus one that will fit the power steering pulley nut in order to turn the
engine over. You also need a set of feeler gauges. Istead of repeating what
has already been written, try a google search - here is the result of one
for Google Groups:

http://groups.google.ca/groups?hl=en&lr=&ie=UTF-8&threadm=3966D389.3FBA0BDC%40ho
me.com&rnum=2&prev=/groups%3Fq%3Dadjust%2520valves%2520mercedes%2520300d%2520%26
hl%3Den%26lr%3D%26ie%3DUTF-8%26sa%3DN%26tab%3Dwg

You could get a garage to do it for you, but make sure they do it with the
engine cold and that they use the proper clearances.

Make sure you ionstall a new valve cover gasket and make sure it is
properly seated (It's a bit of a trick to get the cover back on without
dislodging the gasket.

Adjusting the ALDA is a job that many garages either don't know about or
won't do. It is very simple, but there is a small risk that you might damage
the ALDA. Again, instead of repeating the method, here is the result of a
Google Groups search:

http://groups.google.ca/groups?hl=en&lr=&ie=UTF-8&selm=pBoRa.8252%24Je.3570%40fe
d1read04

Cleaning the banjo fitting and the pressure tube from the manifold plus the
ALDA adjustment can do wonders for a sluggish 300D. Also, make sure none of
the ruber connectors on the pressure tube are leaking - the ones on the
firewall switch sometimes crack.

If those links don't work try your own search.

If you want to get into working on your own MB, there are a few MB mailing
lists that could be helpful if you join.

Good Luck

Less sulfur in the fuel.