Just for the sake of being contrary...of course there are limitless off-road, dirt-road, mud-road, sand-road, plus boulder, bedrock and gravel-road possibilities throughout Central America. Same is true between Portland and the Mexican border. You need to decide where you're going to be going, then choose your bikes accordingly.
If you're staying on streets and main highways, any old bike will do fine. If you'll be poking around a bit you'll want something suited to off-road at times. Personally, with three months to do the trip I'd want something at least nominally off-roadable. There's too much to see and experience to limit yourself to main highways.
And really: "the crappiest bit of road in CA" is as crappy as any road anywhere--and it's not part of the PanAm highway. Should you find such a road, potholes will be the least of your worries. You'll be traveling during rainy season, when backroads that started out abysmal will be overlain with bottomless, snot-slick mud. Not that there's anything wrong with that.
Again: decide where you want to go, therefore what sort of roads you'll be traveling. Choose bikes, suspensions, aftermarket protection, and tires accordingly.
Hope that's helpful.
Mark
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