In my experience "Deathstar" units tend to underperform. Add to that the very nature of the Ork codex which relies on multiple redundancy and a backup to the backup plan and Deathstar units seem even more unimpressive as it takes away from the army overall. Of course that's just my standpoint.
I think the best bet for your tourney would not be a Deathstar unit of 10 Megas in a Battlewagon, but rather 2 units of 5 each in a Wagon possibly with a Mega Armored Boss. That way instead of 1 super Ard unit your opponent would have to deal with 2 Termie equivalent units. In fact you do have the possibility of going full Deff Wing with a grand total of 5 possible Meganob units.
Of course you still would have the same general problem that Megas have, which is a high susceptibility to force weapons, Ap2, Instadeath via Str8, Slow and Purposeful, and Ld if a Boss doesn't accompany or is killed in a Challenge. Taking Grotsnik as Skeetergod suggested would help by giving them an Invul via Cybork Bodies, as well as FnP in the unit he's with, but even then he doesn't come with a Bosspole and WILL be targeted by every Precise Shot and Challenge possibility to take out your FnP bonuses.
If you really MUST have a deathstar unit, the best choice would really be a large horde of Biker Nobz with a Dok and 2 attached Biker Bosses. Sure they don't have the 2+ of the Megas, but higher toughness, better guns, faster movement, the constant cover save from the Exhaust Cloud Special Rule, easy access to Cybork and FnP without taking up an HQ slot, and if you take a Boss or two with them he can separate for suicide runs against AV. Sure it might be a suicide and I call the tactic just that, but in the last edition this Green Torpedo attack was the best way to take out a Land Raider or Monolith, and for the most part I believe it is still viable (although truthfully I haven't done it so far in 6th).
You could also have a unit of 8 Megas with Grotsnik and a Megaboss in a Wagon, it's the strongest and Ard-est that Megas can get. It'd be a nice tough Deathstar unit, but frankly I don't think its a wise investment of points or the financial aspect of buying the minis (although konverting them would be great and I'd ask that you supply pictures if you go that route).
Peace
~OD