Hi all, new member here, dealing with a 1999 SL500

Not very familiar with Mercedes (yet) but am hoping someone can provide some advice on this issue I'm faced with. Recently, the SL500 was sold to a friend and the very next day the car will start but as it starts up I think I can hear a backfire into the intake and the idle stays at about 1300RPM. The accelerator pedal, no matter the throttle position, will only rev the car to about 1600RPM in park or in gear - top speed is about 10mph.

I put my Autel code scanner on it and there are no stored DTCs in the engine computer. I looked at the spark angle and it seemed like it may be off but I'm not sure of base timing, so I started looking around for the cam and crank sensors. Ultimately, at the request of the new owner, I went ahead and found a crank sensor locally and installed it. Not sure of Merc, but I think the crank sensor starts the car and the cam sensor controls timing, but I could be wrong. Most of my cars are so old they don't even have cam sensors, so I feel like that was probably a waste of time - especially because it didn't fix anything.

So then I unplugged the air mass meter and the engine condition definitely got worse so I figure the AMM is probably functioning, plus, no codes for it. After further research, I began seeing folks having issues with the throttle body actuator. Is there a way to diagnose and/or rule it out as a possibility? The new owner wants me to order a cam sensor and try it, but I don't think firing the parts cannon at random is going to fix this one.

It's worth mentioning the ESP/BAS light is on and the brake lights work so I assume the brake switch if good. Also, the new owner removed the hard top and put up the soft top the night before this all started happening. I believe the ESP/BAS light was already on prior to the engine issue.

Thanks in advance for any input you may have and if there's any further info I can provide I'd be happy to.

Small edit; my scan tool is unable to analyze fuel pressure on this car (that cell is blank) but I feel there is sufficient pressure to operate the car since it idles high and refuses to idle down once warm. The more I think about it the more I'm leaning to the throttle actuator. I wish I could get more accurate timing information and pitch it against published numbers. The timing angle cell showed -23 at fast idle and -28 at WOT (WOT is still under 2000rpm - engine will not accelerate past that). Not sure why it's showing a "-" in front of the numbers..