I think initially you need to run a couple of tanks of high octane gas thru the vehicle, adding Techron to each tank won't hurt and might help. Driving two tanks of gas thru will hopefully make the tires round and decrease any flat spots, get a good 10-15 dollar Tire gauge and check your air pressure to insure that is within the correct recommendations, then if the dashboard tells you at some point in time that a tire is low, you know when the tires were correct and how long it took for a tire to lose air. Gasoline changes with time, because it evaporates and the additives added to it tend to decrease and significantly alter its properties, so getting rid of the old and replacing with new is a good thing. Oil does not change with time it only changes with usage, if you can't tell when the last oil change was you should put it on your list of things to do, but another 1000-2000 miles or 60-120 days probably won't make a huge difference. Brake fluid changes with time, because it absorbs moisture from the air and also aluminum components create contaminates that cause it to turn black and it hydraulic abilities decrease, so if the the fluid is black add changing to your list of things to do, but once again the same difference in mileage or days as with oil probably won't make a significant difference. Same comments for the transmission fluid, however it is more than likely that the electrical connection harness has a small fluid leak and when you change the transmission fluid you should probably change the 10 dollar connector part and kill two birds with one stone. Buy a 5 dollar battery monitor and plug it into the cigarette lighter if it shows about 14.5 volts when the alternator is running and between 11.5 to 13 volts when the car is not running and you don't have any dash lights indicating electrical problems, your battery is O.K. for now. Before you take it to a Mercedes mechanic and spend 2-300 dollars on a diagnostic computer check, spend 15-20 on a Onboard Computer Diagnostic OBD device to see if you have any codes that a cheap device will show (do a search on this forum for lots more info), you could also spend 400 dollars on a Mercedes diagnostic system (do a search on this forum for lots more info) if you are so inclined. One of the reasons many of us buy an SLK is so that we can work on then, however only things that keep you from driving it, need immediate attention.
Check that all the fluids are at the levels that are correct.Clean the MAF: might help with the sluggishness. (do a search on this forum)
Think about new sparkplugs: might help with....
Do the gas pedal reset: might .....
Run the steering wheel lock to lock
2 tanks of high octane: might help....
New engine air filters: might......
Cabin filter, (do a search....)
Transmission fluid change: might.....
Oil change, you won't notice a difference, good thing for the longevity of the car.
Brake fluid change, you probably will notice a slight difference, good thing for safety.
Battery change, you probably won't notice a difference, unless the old battery conks out when your 50 miles from nowhere.
Fresh engine oil takes forever to significantly degrade in a good condition engine.
But if it was old/used oil different factors come into play.
Old oil can become acidic, carry sludge/particulates.
These particulates will likely be carbon (from combustion products), or metallic (from poor lubrication of metal to metal components - most should be caught by the magnet in the sump plug).
This can then cause blockage of lubrication channels.
The typical shelf life where oil continues to meet is specification is given as 5 years,
although it can last longer.
It's a minimum value.
The simpler the oil blend, the longer it stores. More complex = more factors in play.