Hi Sean i have several RPM kits in my stash including the Marder 1, i also have the Lorraine artillery tractor and the Stuks zu fuss and finally i have 2 15cm sdf h 13/1 kits, the only way i can describe these kits is that they need a little work and patience when building them.
Also i would say that if you have built any Russian kits such as Ark/Alan or Maquette etc, then these are very similar, the instructions need a little patience, you will also find a few pieces that will not line up properly, but if you can get past these, then the Marder 1 will be an excellent kit to build.
Not bad kits in my opinion, but just remember........ that they need a little more patience in building as say the more up market kits such as Dragon, Tamiya etc
Hope this helps.
Tom