Volt-amp and amp are different. You can argue the technical aspects, but I think VA is often used just because of the higher numbers involved; the numbers seem more impressive.
Ten amp is big transformer. I'd say it's "what you want" more so than what you need (minimalist need). But buy big and the future is ready for "anything" 
Ac will tend to run cooler than dc. Everything, even wire, gets a tiny break from ac's rising and falling of voltage each cycle (60hz.. 60 cycles per second) Dc is a constant push, not much rise and fall of voltage beyond some latent "bumps" from flipping the negative ac wave and smoothing all the voltage peaks with a capacitor. (batteries have no lumps... pure dc)
A Lionel ZW, KW or A.F. 18b, 32b should be plenty for postwar A.F. (with bridge rectifier used for your dc trains..$5-ish, over rate it to near double for a cool, non-cooking, long lasting component ).
