1. People who talk about Dair as the only PC option are the same people who said Brawl would destroy the Melee community. Remember that. (Yes, I am being sassy. I do this a lot.)
2. Retreating Bair is godly. Using it offensively is dangerous, because in order to attack again you have to do the same attack again or turn around. Not that those are bad options, they really aren't. The main reason I myself don't use a Bair to approach is because there are better options, like run-up shield, run-up jab, Fair, Nair, SHAD, etc. Think of it as a gimmick move like gimping followed by a turn-around Fsmash.
3. The only way to get the spacing for Nair is to just use it all the time. If you didn't hit, get closer. If you get shield grabbed, move back. What I actually did (and don't do this unless you think it'll help you) is I went into training mode and simply used just about nothing but Nair for approaching. I tested exactly where on the animation Nair hit and what it looked like, and then I put the computer on and tested it. When I got too close or got shield grabbed a lot, I turned the computer off and went back to training. This sort of thing helped me.
Now I'm not knocking Fair, it's a good approach. But use it a ton and you've lost your unpredictability. Plus it's slower than Nair and has higher landing lag, even if it's a massive spacing tool. Understand that Nair is faster, has less landing lag, and is still a very save move though less safe than Fair. The point here is that Fair is the huge cushion you only use when you need, while Nair is the perfect little cushion which suits all your needs.