#1 You can buy a manual for your car to help with parts identification, ebay is a easy place to buy one, the manuals come on CD's and run anywhere from $10.00 on up. #2. The tube that connects to the air cleaner housing is the intake hose even if its cracked it is not the reason the air cleaner assembly rattles, grab the metal housing itself and see if it moves, if it does chances are the air cleaner mount or the rubber bonded metal shock absorbers are bad or a combination of both and will have to be replaced. #3 Old cars can leak a little, Mercedes is not excluded, you will have to get under the car to determine where it is leaking from, it is best to start off with a clean motor and under carriage then it will be easier to find the leak or leaks and also decide if its something reasonable like a fuel or oil line, pan gasket or something more difficult like a motor or trans seal. #4 Diesel fuel leakage, check all the fuel lines and around the the pump and injectors for moisture (fuel) replace hoses and tighten fittings as required. #5 power locks, what line did the previous owner cut vacuum or electrical?, in the trunk next to the spare tire is the vacuum pump for the door locks does it operate? if so find where the vacuum line is cut and repair it, maybe the locks will operate. #6 power seats, at least the motors operate, so that takes the motors and switches out of the loop, the seats have drive cables from the motors to gear drives, early Mercedes sometimes have drive cables that were to short from the factory, you have a few options, buy and install new drive cables, modify the drive cable housing for the non operating mode (up/down), I have had good luck with disconnecting the cable from the motor side and sliding the shaft inside to re-engage the gear drive, which on the last one i've done has been operational over a year now, or you can leave it non operational, btw i did not have to remove the seats to fix that problem, only remove the four bolts holding the seat and tip it back. #7 tranny problem? are you sure its the modulator? check and renew all vacuum lines as needed, it doesn't have to be much of a leak to effect motor or tranny operation also has the tranny been serviced lately? #8 cruise control, it could be simple like a loose or corroded connection or fuse, sometimes accessories that share a fuse will operate and others will not, then you replace the fuse and everything works, it could be the cruise control amplifier (more difficult) either rebuild or replace, there is one on ebay now for less than $25.00 last time I checked. You have enough to keep you or someone busy for a few weekends, good luck and welcome to the board, alot of knowlege is here, just ask or do a search.