Maybe this is where I differ with the more collector style people in the space, but my 12v collection is dictated mostly by the incomplete sets I've purchased.
For example, my current collection is a bunch of 4.5v system stuff, some of which are official sets and others own builds. 12v being then a 4 Car 7745, a 7745 B model steam engine, a 7740 and a 7821.
I've rarely set out to get boxes for example, so 7860s I've amassed through mostly come across the isolator rails in lots, and then buying the required remote controls for them.
Just my two pence, I've just found there are cheaper (and often more rewarding!) ways of getting these old sets than competing with the whole internet on eBay.