When I bought my 2009 car I too was concerned about muck traps and possible hidden corrosion, so what I did was clean the underside with a lawn sprinkler (nothing but minor surface rust) and apply some rust protection (Dynax and ACF-50). I also removed the wheel arch liners to see what the state of things was under there. I doubt they'd ever been removed before, as there was a lot of built up mud/muck, especially on the rear. I cleaned all this off with one of those plastic/rubber brush drill attachments and was pleasantly surprised to find the paintwork underneath to be like new. I then liberally spayed these areas with Dynax before refitting the liners. Once a year (just before winter) I spray the underside with ACF 50 and apply some corrosion block grease to the inside lip of the wheel arches.
So, in my exprerience, the rustproofing seemed pretty good compared to my previous car (Jaguar).
Like you say, prevention is easier and cheaper than cure, so I'd get a good look at as much as you can, clean it up and apply some rustproofing. It's also worth having something like clear nail varnish to hand, to apply to any stone chips, to prevent them from rusting until you get round to a repair.