"If I can help it" means if I can avoid it, if I have another option. "Not" is short for the previous phrase (which we don't need to repeat) "I would not go traveling without any preparation," so the whole thing would be "I would not go traveling without any preparation if I had another option."

'Not if I can help it' means something like 'Not if I can avoid it'. Your sentence is OK, but perhaps slightly (but only slightly) odd, as you have sort-of doubled up on the 'doing something' idea (preparation and avoiding) "Will you being going to her party? Not if I can avoid it" / "Will you take a slow train? Not if I can avoid it". I'm not sure 'not if' is really a logical grouping. "What about the traffic? I will avoid it if I can" "I will avoid her if I can"