Damn you got some nice luck to pull legendary out of gacha All I ever got during that month and half+- I am playing was bunch of blue ones I already have and purple Beatrix who isnt half bad but gets a lot of heat cause lots of stages have bunch of fire element enemies :/
Anyway definitely get multi build when you got enough gems.
I am not playing very long, buildings mostly around lvl 6 and already taking days to build. Even though you wont save up money that fast because stages except boss fights dont give much unless you are high vip you will still want to be building more than one. You want to get barracks up as fast as you can which unfortunately needs coin storage at appropriate level as well.