I agree, the versatility of Playmobil is one of the things that sold it to me as a parent, along with my fond memories of playing with my brother's playmobil as a kid. I thought he'd had a lot, but when I visited my parents a few weeks ago the sets I discovered (mostly complete) were the rather large Esso gas station,
Construction Workers and Scaffold,
Forklift, and 3 sheriffs, 2 umbrella girls, 2 farmers and 1 sheriff's horse from MacDonald's along with the mail order MacDonald's teepee/canoe/Indian chief set. Those 4-ish sets were more than enough for hours and days of play and a wide variety of stories.
I love your idea of the minimalist Greek myth set, tlc -- as a not-so-minimalist I'd want more optional add-ons male and female clickies so that you could have a great big family dinner with all the gods or, sticking to the myths, you could have Hera, Athena and Aphrodite racing for the golden apple. But I would definitely want to buy the set you describe.
And I share your concern about instilling a more-is-more philosophy, I'm also trying to avoid having so many toys that my son's overwhelmed by them. Several of the sets I've got for him because they were getting discontinued and were on sale are hidden in a closet. I haven't come up with totally formal purchasing guidelines but yours work for me, except for the portability one, though I am considering getting the puppet show one partly because of its portability.
Other things that have influenced purchasing decisions since I started buying Playmobil for my son (but sometimes he's just the excuse) back in October:
- I'd rather buy safari animals than zoo animals, which is the only reason we don't have any penguins yet. Wild animals in captivity make me sad (sorry, bonniebeth!), so I feel a bit guilty about the baby elephant circus set I bought even though it was so discounted I couldn't not get it, and I won't be getting the Animal Trainer Set 4233
- He got the ambulance partly because there was a female paramedic, and I picked up some figures from Craigslist primarily because there were a mix of both men and women who were dressed for jobs and men and women who were more just family figures
- Any sets with guns have the guns removed for now before he plays with them, he's only three.
- Fun sets that encourage discussion of how modern life works (recycling truck, plane) are great for now. I've also got some sets stored away that encourage cultural/historical knowledge: the roman arena and some supporting roman characters, and I'm probably going to get the pyramid at some point.
- Desired props for creating stop motion animation/music videos in the future are hard to resist (this one is pretty much entirely self-serving), as are sets that align strongly with his interests (fire trucks, recycling trucks, Cinderella -- I'm still on the lookout for that one.)