Is it normal for car error codes to change?

Some advice would be really grateful. I had to get my car towed this week as it wasn't accelerating properly and felt like it would die in the road. The breakdown service gave me an error code of p0068 (MAP/MAF).

I had my car taken to an autocentre for a diagnostic check who then told me the other code had cleared and it was now showing p0301 (misfire in cylinder 1). They are not equipped to explore any further or repair and suggested i take it to a Mercedes specialist.

Is this normal? I was hoping it would be a simple fix but the car (Merc B) is 20 years old and I'm fed up of throwing money at it these past few years.

Apparently its £500 to change spark plugs, if that's the problem, which seems excessive considering how cheap they are, but I guess its labour intensive.

Any thoughts?