A group buy is starting on conical flasks here (http://www.nationalhomebrewclub.com/cgi-bin/yabb2/YaBB.pl?num=1354614087). If anybody is planning to do Lagers in the future,you will need to produce a large amount of yeast,so the 3L flasks will be for them. Forward planning and all dat.
In the future I hope to have slants of yeast, would I need to get all of the flasks to be able to step up? (from slant to 2-3 litres)
I don't see any reason why you couldn't do everything with a small one (say 250/500ml) and a big one.
So 500ml and 2l should do the trick?
Yeah that will be fine. The 500ml and 2L/3L is the most popular combination on the buy so I think most people are thinking along the same lines as yourself.