i think the guy you bought the car from is a muppet. the button on the bottom of your accelerator is the kickdown switch which is present in almost all auto mercs from the 70's.
as for the noise it seems it's just the noise from the engine which sadly is quite a roarer. as mentioned earlier the car is most likely in "S" mode and this is why it does not wish to change gear.
in 2000 (if i'm not mistaken) the selector next to the gear should be "S" and "W", which stand for Summer and Winter. "S" starts in 1st gear and needs revs to switch to the next gear (meaning it holds gears longer) whereas "W" mode starts in 2nd gear and switches gears much earlier on in the rev bend.
what you are describing is that you car is not shifting gears correctly and for that you need to take it to MB or any qualified MB mechanic. there is a silver pipe coming into your engine bay from the gearbox and it has a red plastic cap. that it to adjust your gearbox, do NOT attempt to do it yourself whatever you do. let a professional handle this.
Ishoudl point out that even though MB claim that their gearboxes are "sealed for life" it is advisable that you get the transmission fluid changed every 60,000 and if your car hasn't had this done (or you are not sure) and you have crossed that milage you shoudl get it done ASAP or you can run into alot more problems.
sometimes you will hear a strange mechanical humming noise from under the rear seat, that is normal for some model mercs to do and has to do with the charging system.
let me know if there is anything else i can help you out with at any time.
good luck and happy driving.
(please note the in this model C-class the buttons may still be "S" & "E" which stand for sport and economy modes but the principle is the same as "S" & "W" modes)