Hey, you're in my old Div V league. Congrats on a great year! Do the AG Wizards still have that tough Center?
I had the same concerns when I promoted, and so right away bought a veteran SG (Bratosa) who helped me become a little bit more competitive in what was a fairly tough Div IV league. I would have probably been in the relegation tourney or barely scraped my way into 5th place, but several teams went bot during that season, so I actually made the playoffs (only to get thumped brutally in the Conference Finals by the eventual champs).
Like the other posts said, definitely expand your arena. If you win the championship, you're going to get a huge attendance boost through the first 15 games or so of next season, so more seats are great and also you can charge more for tickets. If you lose in the playoffs, you'll promote with the same promotion bonus of $200K, but apparently won't get that same attendance boost (I may be in this boat as well this season).
This has also been stated already, but it's worth it to wait and see how tough your league is before enhancing your roster. You've got some solid players already. Your bench could use some improvement, but that will automatically happen as you bring in a couple new starters and your current starters become reserves.
Best of luck.