To clarify, you can't get back the characters you had; the server has no access to the account data.
But almost everything that was unlocked in the Paragon Market or Veteran Rewards is either given immediately to the player, or is in-game vendors for a reasonably small amount of Inf.
All costume pieces are unlocked immediately as well as capes and auras.
If you saved your character data with Sentinel+ before shutdown, you can use this page: https://cit.cohtitan.com/senticon/ to extract the costume data out of them in batch and then you can put the files it creates into the install folder and load them into the game and immediately get their costumes back.
As all of the 'Pay for' costume slots are unlocked, all characters start with 6 costume slots (including the mandatory one) with 4 more they can unlock via game-play (as usual).
If your costumes have different skeletons and heights (due to the Science Pack) you will have to head to an Icon Surgeon or Facemaker Surgeon to use those costumes.
All changes up to level 10 are Inf free.
The Pay-To-Win vendors (which run on Inf, not real life money or anything like that) will give you for free the Prestige Sprints, Sands of Mu or Undead Slaying Axe, and Nemesis Wand & Blackwand, as well as one of either Ninja Run or Beast Run, and one of the Shield or Power summon pets.
Other things can be bought there for usually 100,000-1,000,000 Inf, which is relatively easy to get due to the new Reward Merit system that's been expanded, and the Auction House has been seeded with basic salvage.
Supergroup Base prestige has been removed and Energy/Control needs for the base have been removed so you can build whatever you want including pieces that were never in the game when it was on Live.
Null the Gull can also change your Alignment between Hero/Vigilante/Rogue/Villain instantly as well.
It doesn't mean you can instantly recover everything you can from the start but it's a lot, lot easier than it was on Live.
If you are absolutely requiring an instant push button to level 50 for characters there are other servers that have that and if you have the hardware and know-how you could run your own too, but no server out there has all your personal data from shut down.
That said, I hope that this helps and that you stick with the server. But even if you find a better fit for your needs, I hope that you can enjoy the game.
