Civilization II has much in common with Civilization, but there are many small differences and some major differences. Players familiar with Civ1 who switch to Civ2 can simply read the Civilopedia and other main menus and the relevant game-specific pages on this wiki, but some details are of little significance whereas others deserve particular note if you want to avoid nasty surprises.
Required before advancing to population level 8, not level 11.
No great anxiety about model 1 or model 2 being obsoleted: you get the money. Model 1 expires with Gunpowder (Civ2) as in Civ1, but model 2 lasts longer.
Never expires. So you should not be afraid to research Automobile (Civ2) or to see other nations moving towards it (with prerequisites Combustion (Civ2) and Steel (Civ2)), unless you have Leonardo's Workshop (Civ2) (in which case you should go for Machine Tools (Civ2) first so that your Cannon (Civ2) upgrade).
Costs more (in proportion to its attack strength) but has double the defense strength and is therefore relatively more valuable. Probably a better buy than the phalanx or archer in most situations.
Much more use than in Civ1, and never expiring: adds a Granary (Civ2) to every one of your cities. The "granary line" shows in each foodbox, but the granary is not shown in your list of buildings.
If you have any real granaries when you either acquire the Pyramids by conquest or, less likely, build that Wonder, you should usually eliminate their maintenance cost by selling them, especially if there is any risk of losing the cities that contain them.
If you have Adam Smith's Trading Co., there is no maintenance cost, so keep the granaries in case you lose the Pyramids, if you are confident of keeping the cities that contain them.