Sounds like you're thinking things through well ahead of time. You can never be too prepared for these trips. Well, you might be too prepared if you have so much gear that won't fit in the wagon
I was going to say, in terms of pre-trip maintenance, wheel bearings are a must. Even more so if you don't know the last time they were repacked or replaced. My cousin had a front wheel bearing go out on the Alcan -- NOT fun.
You seem pretty confident that you can find parts while on the road. While that may be true in larger cities, I suspect that you'll be traveling through some pretty desolate areas. Even in Canada and Alaska, you'll be several hundred -- maybe even over 1000 miles -- from the nearest MB dealer or well-stocked import parts supplier. There isn't a MB dealer between Edmonton, AB and Fairbanks, AK, for example. I guess I'm saying that in very remote areas, you might be up a creek... That's where a strategic onboard spare parts bin becomes essential.
Re: fuel, jerry cans are convenient. Especially if you can source some mil-spec ones that you can lie flat on the roof.
Unreal that the Darien Gap requires a container for $1-2k. Can't they run a car / passenger ferry for cheaper?
Anyway, I did the Alcan roundtrip in 2009, and several other forum members have done it as well. As you get further along with your planning, I'm sure that we'll have some advice / opinions to share, if that's what you seek.