After 50 years of building I find I've become a collector too. It kind of snuck up on me, and now I'll even buy an empty box if it pushes my nostalgia button. I actually have the nicest boxes in the display cabinet with my built models.
Recently I bought the original (1962?) Monogram 36 Ford, MIB for $24 on US Ebay. This is the kit that really started my obsession when I was 13 and the pristine, unassembled parts, the instructions and the box itself are a joy to own again. I'll resist the urge to build it by building the latest re-issue instead. It's identical except that, like every other re-issue of this kit, it doesn't have stock wheels, tyres or hubcaps. But those from the Revell 37 Ford pickup are a great substitute.