I’m currently fixing up my bike, and I’ve come to a dilemma. For background, I got a bike at a police auction, but the rims were damaged and had a bulge in both. It’s a V-brake system, so it affected braking.
I got a donor bike with better rims, but it used a Sunrace freewheel. Is there a meaningful difference between the Shimano and the Sunrace, other than how the gears attach? Would you recommend going through the effort to swap the hubs?

They are probably compatible according to SunRace’s Q&A https://sunrace.com/faq-qna/:
That said, is the cassette still in good shape? Might want to confirm it’s still got a good amount of life in it before you go to the trouble of swapping it out.
How would I tell if it’s in good shape? That’s not something I currently know how to do (but I’m willing to learn). I can remove gears, and I can grease wheel bearings, but I don’t know how tell the difference between a worn cassette and a new one.
Compare how the teeth look on yours vs a new one, you can get an actual cassette wear tool but you should be able to see how warn down it is visually.
Feel free to post pics too for second opinions.