I wouldn't use a plat armor on her unless you have 3x black fairies. Otherwise the armor is going to waste. And then you would still need a 4th black fairy to CC her with.
It really is not as bad as you think it is to level 40 to 50 without a plat armor. I do it ALL the time because it saves me plat armors for later. For Nanaly, she gets a bonus exp from archer fodder, so DH is a really good place to grind for her. In all honesty, once you get to DH, you can grind that and not care about your gold any more. You'll be fine, I promise. As long as you don't try to level units past 50 with only the gold from it. And if it really makes you feel better, then the gold boost in the shop is really not a bad deal if you suffer from empty coffers. It takes the DH reward up to 5,400 gold per run.
Make Nanaly to 50 your priority. If you don't already have one, then work on Tuesday/Friday dailies until you get at least 1 black spirit. And then use that and 3x silver archers to CC her. Level her up to at least level 40 again after that, and just watch the devastation with her skill activated. You won't regret it. You'll just be mad at yourself for not having done this sooner.
I'll say this again just to be clear. Don't worry about your gold. Just spend it to level up units without using plat armors. You will be fine if you just grind DH. If you grind anything else then I won't make the same promise.