For Carriers and Light Carriers, there thankfully aren't many worthwhile retrofits available. Really there're just Ranger, Shouhou, and Saratoga (whose retrofit we don't have yet). And Ranger and Shouhou are relatively cheap to retrofit, so have at it.
Cruisers (both light and heavy) and Destroyers have a FUCK TON of girls who are worth retrofitting, so you're gonna need to decide how to budget your retrofit blueprints. You get blueprints from doing hard mode of the main story quests, and you only get 6 clears of hard mode per day. Maps that end in -1 give Destroyer blueprints (so for example, 1-1, 2-1, etc), -2 give Cruiser blueprints, -3 give Battleship ones, and -4 give Carrier ones.
Not too many battleships worth retrofitting, and 4 of them actually totally change class if you retrofit them. Hyuuga, Ise, Yamashiro, and Fusou transform into BBVs (Aviation Battleship) if you retrofit them. General consensus is you totally gimp Yamashiro and Fusou if you retrofit them completely, so what people do is they fill the retrofit nodes in besides the very last one, so they get all the stat boosts but don't class change. Hyuuga and Ise are actually great even after becoming BBVs, but there're still trade offs. We don't have Hyuuga's and Ise's retrofits yet, but we do have Yamashiro's and Fusou's.
As for limit break mats, there's no way to "farm" them. You can buy them with merit from pvp, medals from retiring ships, doing the weekly tasks, and collecting them in the monthly check-in thing. Even if you lose every single pvp match for a season, you should still get enough merit to buy at least one gold Bulin from the merit shop AS LONG AS you actually bother to do all your matches.
EDIT: I was wrong on the last thing. You need a win ratio of around 50% for the season at least to make enough merit to afford a gold Bulin for 15,000 merit.