You definitely won't be able to win on the upper difficulties and on the larger maps without building the lower tier units. Really, you should never stop building them at all during the game. The reason is the per-ship-per-level unit caps. Unlike most rts games, you cannot just spam the highest version of your favorite unit. You need to build all of your unlocked units to get the largest possible fleets, and then the trick is deciding how to use the outdated stuff (as a real military commander has to do).
Some players always put the lower tier stuff on homeworld defense. I tend to keep it mixed with my main fleet. The extra firepower is handy, and they absorb shots that would otherwise hit higher-level ships. When there is an ion cannon that makes me have to leave my lower level ships behind for a given raid, that is a real blow to my offensive effectiveness unitl that thing is dead.
Regarding defenses, pretty much everyone unlocks some sort of more turrts. They are better in a lot of ways than the mobile ships, except they are immobile and require supply. Which turrets you unlock are up to you and your current situation. I don't unlock a ton of defensive stuff myself, but that varies by player. I tend to always get transports and rally posts, and mobile repair stations are also super popular. Higher level force fields can provide needed security, but it depends on the game if you need them. Same with fortresses, though I think they are less common by far for most players.
