For any non factory anti-theft Mercedes without traction control, we would do a complex alarm and remote start with full features and top-line gear (good wiring, solder joints, brain buried DEEP, remote start with full safety, etc) for about 12-15 hours labour.
Final bill would be about $7-800 for labour. But, perfection costs money.
Anyone quoting you much less than $3-400 is going to do a hack job.
And don't forget that the wiring diagram doesn't show WHERE the wires are, just what they're connected to. Take LOTS of time to examine it and learn your way around.
1998 C230 "Black Betty"