I have always thought rarity referred to the number of cards in existence, while scarcity referred to the availability of the card. A rare card will probably always be scarce, but a scarce card does not necessarily have to be rare.
T209-1 is my favorite set, and was the first prewar set I completed. I think the pop numbers are somewhat inaccurate, as I recall seeing numerous examples of T209s getting cracked and reslabbed by different TPGs. Some of the cards are certainly tougher than others. Hoffman is probably the easiest, and Booles is probably the hardest. I think I've only seen 2 Booles cards come up for sale in the past seven years, and I bought one of them. So as Ted hinted, putting together the set will take some time and patience. But it is doable, especially because it's only 3% of the size of the Monster!
ETA: About seven years ago, I took substantial notes on the PSA and SGC pop reports for T209-1. At that time, there were 398 total cards slabbed by both companies. So that number has gone up by about 100 since then. The hardest card was Booles with 17 total examples, and the easiest was Hoffman with 38.
Here is my set:
