This depends. If you are tracking with 'phones, this can be OK to listen for artifacts (hums, buzzes, ticks, etc.) that you don't want to have ending up on the track. But for mixing? NO. Headphones don't work well for that , they exaggerate your stereo field, as well as hype various frequency bands, and this will cause you to make wrong decisions in the mixing process. You really need to treat your listening space and use decent, playback speakers to mix.

The short answer, no. You still need to have a decent pair of monitors as a reference even if your acoustics are crappy. You wouldn't be the first control room to have crappy acoustics. Trying to mix through just headphones is an exercise in futility and amateur hour. That is, until you learn how to do it properly by also utilizing control room monitors as a primary reference. So when you have lousy acoustics and a pair of control room monitors, you start to listen to all of the most incredible CDs from the most incredible artists, from the most incredible engineers you know. That becomes your base reference. Without a base reference, your crappy acoustics will override any of your sensibilities & perceptions. Finding your sweet spot may be difficult. That's another reason why you need your base reference CDs by the powerhouses of the industry. If their stuff sounds right on your monitors and your stuff sounds crappy, it ain't the acoustics. But it's easy to blame the acoustics. A common mistake by entry-level folks and nothing to be ashamed about. It's a learning curve. And that learning curve ain't flat like a frequency response plot. It's ongoing for all of us no matter how long you've been in the business. I'm still learning at +40 years into the career. And this is all I've ever done. Well I did work as a bill collector and PEP Boys when I was 18 for couple of months.

You should have at least 100 W per channel available for a startup. You can never have too powerful an amplifier regardless of what the speaker's ratings might be. You can have a too underpowered amplifier which will blow your speakers up faster than an overpowered amplifier would. That might be difficult to understand? But that's because, if you're amplifier is underpowered and you overdrive it, it will produce spurious harmonic distortion that will heat up a Tweeter faster than your toaster and boom, no more Tweeter. Conversely, if your speakers are rated at only accepting 50 W RMS, you can be pretty certain that they will accommodate more than 150 W of peak energy without blowing out.

I've only blown a couple of tweeters in my entire career
Mx. Remy Ann David

thanks!