Go to shows to learn what speaks to you. At the most recent show I was at there was a complete Lionel O gauge set, pre-1930 with 4-wheel cars, working, original set box, under $200. It spoke to me, and seemed a bargain to me to boot.
Before I could buy it, I was actually thrilled to see a circa 30-year-old buy it first. I'd rather him have it, since my boys of that age have little interest.
As a kid, I drooled over the original Madison cars in my Dad's old 1940 Lionel catalog. But when I see them at shows, either original or repro, the magic just isn't there anymore.
If you go to shows, between your heart and your wallet, you'll know what to buy.