Apart from the Oppo, you’ll want a DAC. I use iFi iDAC2 and iDSD, as do many people. I’m waiting for the Pro-ject Pre Box S2 to come to America, many people are in love with this. These DACs all run around $400-$500. Another DAC coming out is the TEAC NT-505. The TEAC is Roon ready, i.e. no additional endpoint needed. Probably about $1200 on Amazon. You may be one of those people that need a $5000 DAC, but probably not as you have been happy listening to mp3s.
Also, go to Allo.com. There are many good turnkey units with endpoint, DAC, and USB jiggering combined into one unit.
Yes, one can’t string two DACs together and derive any benefit. No, not additive.
I use the iFi iPower wall wart. Don’t know if it makes a difference, but it’s only $50. Or you can just use the standard Pi power supply. Many people will try to steer you to more expensive stuff. Again, are you going to be that much of a connoisseur?
In all things, since you are just starting and have been happy with mp3s, something for which the snobs have unrelenting disdain, start out cheap. You may find that you are happy with that until upgraditis sets in, but by then you will know more about what you want.
Also, see the article in this e-mag -
Oh yeah, another thing occurs to me. There are a lot of Schiit lovers out there. I have a Schiit headphone amp and a cheap Schiit DAC. Look into the Modi Multibit (Mimby) or the Bifrost Multibit (Bimby) before you buy a Gumby.
Beware of some of the advice that would steer you to the more expensive equipment. They’re just giving you what they think is best for them, which is not necessarily best for you. As always, a grain of salt.