Hi Wacky.
The Marder was essentially an ambush weapon and when deployed for action it would be on its own, at least far enough away from its unit counterparts to make the dio far too big. To depict more than one vehicle together in a reasonable size dio, it would need to show the Marder being resupplied (as mentioned above), broken down, etc. Another option is for it to be passing a broken down / knocked out or abandoned enemy vehicle. Remember a dio should tell a story, not just show off a couple of models.
I presume you have done the old Tamiya kit and unless you have spent copious funds on aftermarket parts, you wouldn't want to spend a lot on the secondary vehicle. The cost of the vehicle might be your starting point.
If you want the Marder to be the main focus of the dio, keep it higher than the other vehicles (on a ridge, or on a road that is slightly higher than the adjacent field) and keep the secondary vehicle smaller than the Marder - J-Hulk's Kubel idea or maybe a schwimmwagon would be ideal, or even the old Tamiya KAZ Russian jeep (is it still available?); even a Kettenrad might be okay, but probably not if you want to use the BMW or Zundaf as well.
If you are going to use a bigger model, say an Opel truck/or Steyr RSO for resupply , have the truck offset to the rear of the Marder and to the back of the dio to keep the Marder as the main focus. Same goes for an abandoned tank or such.
Have a trawl through the dio forums for ideas on the placement of the vehicles to give balance. The net also has a lot of places to view dio's.
Have fun with the build, and don't forget to post the pics if you can.