For scale you're looking for 28mm-32mm stuff.
I think that's O gauge for model trains & about 1/35th-1/32nd scale model wise. Someone will correct me if I'm wrong.
The amount & types of terrain used will directly impact any game that uses rank & file units of infantry - be it WHFB, Napoleonics, Ancients, etc. The more you use, the harder it becomes (sometimes with serious rules interactions) to manoeuvre units in those types of ganes. It's much less an issue with skirmish type units - like what AoS or 40k uses.
Beyond creating a table that all players are happy enough with, there is no "right" amount of terrain. Well, unless you're re-creating something historical/well known.
Sources: There's sooo many companies making this stuff.... Just do a general search for what your looking for. And then, unless it's crazy expensive, just buy what you like best. Because as long as you play games in this scale you'll nearly always be able to use the buildings.