-spd/+def Nowi build?

These questions can not be answered in the vacuum you pose them in. What is Nowi's role on your team? Is she just the physical wall that needs to check reds/blues/falchihon users? Do you need her to facetank strong red mages as well?

You need to ask yourself wether or nor boosted resistance will enable nowi to fulfill other roles on your team. For instance, I have been running a double dragon core (Fae+Nowi) long before 2.0, as they are somewhat easy to acquire distant counter units that deal magic damage, which is strong anti-meta. I found myself wanting my Fae to be able to deal with both Reinhardt as well as B!Lyn so often, seeing as Nowi has to tank strong reds (and usually ends up too hurt), that I wound up using +def-HP Fae, and giving her Lightning Breath+ Def. Now she laughs at both of them, whilst ORKOing in return.

+Def/-Spd is undoubtedly her best IV set for SB+QR, and is in itself more than worth killing off an Ike for.

Whether you want to go for her or BK is purely a question of what you need; Nowi is a more general tank who can fit into practically any mixed comp, and thus I would argue is overall better, however BK is the superior choice if you already have a competent Armour Emblem sorted (or in the works).

Which LB++ is a throw-up, and it's purely up to you - Def makes her more of a hard counter to archers, and allows her to tank Falchion users better (which we may well see more of now thanks to the buffs), whereas Res makes her a better overall tank.

Well. Why does it come as a surprise that +Def/-Spd can potentially be her optimal build? With Steady Breath and added Def you want Nowi to be doubled. The only concern here is how easy it is for Nowi to get quad since Spd is her super bane. The rational with -Spd is that even the neutral 27 is still insufficient for avoiding quad or common doubles. Thus compare to neutral Spd, -Spd doesn't lose much outside of one point of BST for arena scoring -- trivial since Nowi is bound to be +10. So you probably either want +Spd (and -Atk, you can't take -HP) or -Spd (and +Def). The trade off is between avoiding quad / avoiding double aginst axe units and much better tankiness against other common threats.

As for your B!Ike, Steady Breath runs really well on units with DC built in their weapon. So Nowi is one of them. There aren't many others. BK can't be merged and is thus off limit from arena (at least for now). Even with BK, you just don't need that many B!Ike. So in the long run, assuming you likely already have a few B!Ike or will end up getting a few, it is worth giving Nowi (also assuming you like her) Steady Beath.

We have the same dilemma—albeit in my case—I don't have a single Bike, which then I resorted to using TA3 for budget purposes.

There are many ways to approach building a Nowi, but there's only one feasible way for a -Spd IV, and that is a +Def boon. In this case, you should dedicate your Nowi to be a non-green physical check, and leave the other checks to your other units.

Steady Breath is probably optimal given how it perfectly synergizes with your role (and also why Nowi can be SS tier), but if you really want to keep your Steady Breath for something else, you may inherit Fury 3, TA3, Steady Stance 3, or even Close Def 3.

On the choice of Breath+, I suggest +Def. There's no other choice, since you're trying to improve on what Nowi does with her role, instead trying to be a jack-of-all-trades and horribly fail once you encounter heavily min-maxed enemy checks.

I'm currently building a +spd/-res Nowi, and gave her a +spd Lightning Breath. With Fury and speed seal, she's sitting at 41 speed at +4 merges. Is it ideal? Probably not, seems like the steady breath build is pretty legit from the simulator. But I favor speed in this game whenever I can get it, and a 41 speed Nowi sounds crazy. I've just started building her, after holding onto the 4* ones I've accumulated over time and saved up a bunch of feathers. The dragon buff with weapon refinery has motivated me to finally build her up, as I think she has quite a few styles you can go with.

I do have an Ike that I've held onto forever, as well as a -spd/+def 4* Nowi, but I'm not sure if I'll pull the trigger on that build. I actually went with a +spd/-res Ike for the one I actually use.