As far as "Anfetten des Gemisches "..
Yes,it WILL have an effect and NO..you do NOT need an "abgastester".
What you DO need..is a wideband oxygen sensor "anlage".
These days..if you´re into modifying units at all..
Such a setup isn´t an arm and a leg anymore.
Problem with a wideband unit is that it in contrast to a common narrow band needs a converter to run.
The ones commercially available normally also sports an outlet for a common laptop-apart from an LCD display that tells you direct air/fuel ratio(AFR).
You simply adjust for instance the EHA and CO setting against the WB.
For a naturally aspirated engine you tend to shoot for an AFR of 13.1:1 or thereabouts at wide open throttle(WOT).
For a "gemish" that translates into approx 1%CO per the OEM 16V Mercedes specs you touch down at approx 14.0-14.2 AFR.
To check for current pricing of unit...
Check
www.etapp.se and check for what in swedish is called an "Bredbandslambdamätare".
Converter in swedish=styrenhet