Finding the right set up is all about trial & error. That applies to anti roll bars, shock oil weight, spring rates, gearing, tyres and to a certain degree the exhaust. Best way is to get the standard car up and running as sweet as possible then add or change things once you get used to how it performs.
One thing the marder suffers from is grip roll. If you go into a corner with any kind of speed on a high grip surface such as a field, those spiked tyres will dig in and flip it over. I fitted a front anti roll bar (against all advice on the forums!) and found it reduced the effect and suited my driving style and where I ran my car (the 4wd kit really transforms the car and pretty much eliminates grip roll)
Again with shock oil, this is such a variable that no-one can give you the exact weight of oil and say there you go that is the best. I took advice from Mike1970 who races FG's and he pointed me to a shock oil which was 'a good starting point' for trial & error testing. As it turned out he was spot on so I have stuck with it.
Exhaust wise I just run the FG tuned pipe. They are cheap enough and come up on ebay and give a smooth power delivery throughout the range rather than a blistering top or bottom end. The plus side with the bug shell is it fits with virtually no hacking which adds to its appeal for me. Yes there are better pipes out there but this suits me, for now!
Mike1970 posted up some basic geometry settings on here a good while back, he may be able to direct you to the post.