there's really no sure answer for your topic question- as your really only as good as you want to be. for example if theres a certain record I want then I get it, however if I know I can't get it and/or don't enjoy playing the level I just play it a handful of times and settle for something atleast halfway decent. Basically, the more you play a level the better your times will be, mainly because you get it memorized and know exactly when to do what.
Edit: just saw your post, here's the times I got on my first try and are good time targets to work for.
City escape: 2:40
Metal harbor: 1:56
Green jungle: 2:00
Pyramid cave: 3:00
Radical highway:2:20(Watch Cybrax's video and you'll get this easy)
Sky rail: 1:00
And about zones, Yellow or orange are good targets, maybe green as said if your just starting out.
- How do you A-rank mission 4 of Route 280? >_>
I think the game is pretty easy on you for this one, I don't remember exactly how much time was left on the countdown timer but on my file select screen it says it took me 3:42 to beat, so aim for 3:42 or below and you should get an A rank.
Not so sure about the rest of the questions, though there's many SA2 players that know much more than me and i'm sure can help you out. and last but not least, welcome to the site and have fun playing SA2- I look forward to seeing you on the charts!