Alright, the front arms are a fragile part but other then the 501x its the same with all 4wd. However, If you turn through the XX4 forum you will see the fixes to make your xx4 just as durable if not more durable than any of the current generation buggies. I had the crashback arms and they were junk, you are much better off making the arm braces out of an old chassis or similar material. These can be seen and copied from the xx4 thread and I believe was originally posted by F N Cuda. My buggy has taken quite a few massive hits that should have destroyed the front arms, but alas the only thing that broke was the front arm brace and at 13 dollars a set of front arms, id rather break the braces.
As for parts go, you should check out losipartshouse.com
They have everyhting you need there.
As for parts shared with the gas truck, maybe a bearing or two??
You are going to really enjoy your xx4 as I do mine. If you have any questions feel free to pm me or ask in the xx4 thread.