The level cap for the game is 30. Upgrading to Legendary will not let you level to 31. After level 30, the game is about skill points from feats and what not and gear. Your new "level" is your Combat Rating.
As for what the different levels of access give you, here is a link that details it.
The way I understand it, if you pay for Legendary and stop, you would fall back to Premium and only lose the things that are different about them, such as going from no cash cap to only being able to use $2000 at a time the rest going into your escrow. I have no idea what happens to items that are in inventory/bank slots that are lost.
I haven't updated mine list in a while (since Trigon DLC dropped) but you are welcome to copy it over to your own google docs and mess with it.
https://drive.google.com/folderview?id=0B_u948Wd0xtAMnRIWk9aR3JnVm8&usp=sharing
Go to GlowingBin's Skill Planner
Put in your skills and then you can screencap the entire webpage with an extension like FireShot for firefox.
I also cropped it to make the file smaller.
Or hit generate build code at the bottom and then give me the code so I can load it in. It should be a bunch of 1's and 0's like this...
11111011111001110000010010000000000000100000000
There are three levels of plans. Free, Premium and Legendary. I suggest that you aim for at least Premium, since it is a one-time fee of 5 dollars.
The breakdown of the plans can be found here.
here's a table showing what you can get- http://www.dcuniverseonline.com/free/
From the looks of it, you can't use station cash to pay for monthly subscription fees. However, you can use it to buy the DLCs and have access to them forever.
"The Currency tab of the Inventory displays the currency (Cash and Influence) and any badges the player has accumulated. Badges are obtained through select PvP matches and end-game group content (Raids, etc). Badges are required by certain Vendors to buy high-level gear." Source: http://www.dcuniverseonline.com/en/gameguide/pc/
This included PVP gear.
Credit Cards are not needed for PSN accounts.
You can use PSN cards to fund your PS Wallet.
PSN cards are available at many retailers including Gamestop, Walmart, Target, Best Buy, Sams Club and Walgreens.
You can even purchase PSN codes from online retailers including Amazon: