can i buy a dedicated amp and just plug it into my PC with RCA to 3.5 cable and use the PC as DAC?
Yes, in general there is no problem with that, however you may consider buying some USB dongle (everything <50$ should be good enough most of the time) if your soundcard is crappy.
Yes. You can. But you usually shouldn’t.
I’s what I did when I got started out and my SH-9 sounded so shit I almost sent it back for a refun, thinking it was broken. However when I tried it on a different PC it sounded better. Weird? I, at the time, didn’t think I needed a dac. So I got a good quality but budget DAC and boom. Suddenly SH-9 goes from
“Fuck this shit is broken”
“My god this is amazing!”