Agreed, chainrings are rather pricy compared to the chain and cog in the back.

112 links will/should suffice, yes I run 52/20 with a 112 link chain (needed to shorten it, so 112 is more than enough).

I have Dicta as well, but just got a Sturmey Archer freewheel for another project. There's a world in difference between those two and the price isn't that bad. You should really consider that. I'm always worried that my Dicta will break under load.