Can I just buy SSF4?

I used to have SSF4 with all DLC, including Yun/Yang/Oni/E. Ryu, lost my disc awhile back and lost interest, but I’ve been getting back into FGs with Injustice and decided to pick UMvC3 and SSF4 up.

Since I already had the AE DLC on SSF4 can I just buy a used copy of that instead of AE, stupid question I know but I just wanna make sure no weird patches were put in place in my off time.

Also, off-topic, but is MvC3 still more competitive than SF?

If you still have your Live or PSN account, you’ll be able to queue the download of Super->AE patch as an already purchased DLC. Then you install it, then you queue AE->AE2012 (this one is free), install it and you’re set.
See this screen.

About MvC3 being more competitive than SF, I’m not sure it has ever been the case, and definitely not outside the US. Anyway, you’ll find plenty of competition for both games.