Code clearing? ... disconnect the battery. But, understand, clearing the codes doesn't eliminate the source of the problem... e.g., if the gear recognition switch is bad, a code will be generated as soon as you move the gear selector lever from park.
BTW, re the gear recognition switch... you can always replace the gear selector lever assembly, in toto, in lieu of disassembling the assembly to renew just the switch. Lever assembly replacement is quite simple... but the assy. does cost >$200.
Correct transmission fluid level is important for ALL M-B transmissions. A special AT fluid dipstick (approx. $40?) is required for the 722.6 trans and can be obtained at an M-B dealer. If you receive the vehicle back with no fluid in the trans, you'll be unable to measure the fluid drained (and thus know the approx. amount to replace)... in that case you'll definitely need the dipstick to get you in the correct level ballpark.
As for the final fluid level inspection/adjustment at 80 deg C, you can always take the vehicle to an M-B dealer. You'll likely be charged a minimum of 1 hr labor for fluid temp measurement and level correction; perhaps they'll agree to do it for no charge if you buy all the parts from them.
Run the VMI first, though, before you make any decisions. Lets see if this tranny had any warranty work.