I'm no fan of cheap chargers on any level but I acknowledge in your hypothetical that a decent 3-step charger (I'd maybe go with a known brand, like Battery Tender or something) would make financial sense compared to a more expensive MPPT controller. If you already have a good charger at home then something inexpensive is probably fine used sparingly. Battery life is cumulative of every half charge, over charge and what-not.
I have an Iota DLS-45 with the IQ-4 option that I use to periodically condition my batteries. Lately I've been letting the AGM that I use with the panel float on it but I will run a conditioning cycle on the other batteries, especially the Optima, a few times a year.
Couldn't tell you about the panels in your eBay link. Since they wouldn't be out in the weather I'd be less concerned about their quality. With constant exposure to wind and snow and sun good panels will last longer. Less expensive panels typically use plate glass instead of tempered, which you recognize the safety issues there. They are also more likely to begin clouding over if the glass is cheap. OTOH being banged around in the back of a truck will probably break even good panels and replacing them will hurt more. I'd probably go with the cheaper panels myself, too. Build some sort of hinged case to protect them a little.