At your current stature, I would say go farm manyus and level your girls, especially the 5* ones. Yes, they will definitely cost you a lot (because they cap out at level 60 unevolved compared to 4* and below which caps out at level 50, and their evolution cost and materials are definitely higher), however having them at higher levels will definitely work well in your favor.

From what I've gathered while lurking around the forums, people will recommend you go either Stage 1-3 or Stage 2-1 to farm manyus.
But what you're really after is the Secret Garden they spawn. As for those two stages, they both spawn the Beginner version of the garden - which will at least guarantee you ONE level 20 manyu.

Affection-wise, you can probably go for the 4-5* girls. Since they have more stats as well (and not to mention affection bonus gets doubled once you evolve them). If not, go for whoever you want XD

Dunno about 2* girls but they've been proven useful... well as sacrifice units when specific event maps require so.

You can look for more info on the beginner's Q and A thread, and the previous event threads as well.