well SL doesn't say too much, what matters in my opinion is that a) your main damage scaling stat is at max before the cut (for a mage that would be 50 INT) and b) that you have enough endurance and vita to survive without the need for the Ring of favour and Protection. To be able to use the Dark Wood Grain Ring and run at a good speed, for my mage (17 equip load) that be endurance level @28 which yields in 68 end. Put rest in vita and dex for faster casting and you'll rush through NG+ without any troubles.
I am now on my 5th playthrough with my second char, my SL was @94 when I began NG+ with my mage. NG is the easiest of all playthroughs hence that's the place where I do every area and fight every boss. On NG+ and especially NG++ I just rush to the bosses for souls, the occasional farming here and there. NG is for getting ready to do that with ease, I don't get why people want to start a NG+ as quickly as possible. Take everything in the game has to offer on NG, level your char as far as possible, max out your armor and weapons, get all embers, kindle all bonfires because later on NG+(++...) you wont be able to enjoy it as stress free as on NG.
As for covenants I prefere the Way of Light, simply because I hate being invaded when I'm PvE'ing in human form. If I'm in the mood for some PvP action (which usually happens only once per playthrough^^) I switch to the forest hunters and get summoned about a minute after I've put on the ring. Kill (or be killed), wait a minute, repeat.
About the DLC: it's hard as hell, I'd say Artorias and Manus are about the hardest bosses in the game, Manus has the highest HP. Even on NG I had to use the TCC and Power Within to kill Manus, though that's probably also because my reaction is getting slower the older I get. Chances are big that you'll skip them entirely after your 100th fail on NG+ if you didn't prepare your char well enough in NG.
That being said: that's only my opinion and the way I like to play DS. It's a hard enough challenge how it is and I don't see why I have to make it harder for myself than necessary