What you average on 3x3 doesn't correlate with what you average on mega. For example, I average sub-11 and prob average under a minute for mega. The quality of the mega solves + the speed that I allow myself to have whilst doing lookahead is how I got there.
On
www.thecubeskills.com, you can find a megaminx tutorial series that teaches how to solve a mega, how to get faster, methods for solving S2L (Yu Da-Hyun, Westlund, or Balint), and techniques on how to get faster. It also has a good guide on 4LLL.
When doing megaminx in general, you want to be mostly doing solves without the timer. This way, you don't have the pressure of timing yourself, and it allows you to find new stuff with the mega.
As for a good mega, the YJ Yuhu Megaminx V2 will be your best friend. I would recommend getting the black side, although do not switch grey with black right away. If you do find that the grey side makes it harder to determine cases, however, then do switch the colors. It might take some effort, but it's not that hard to do.