(Note that this is largely my own opinion.)
I'm not sure why Ars Magica 2 is there, seeing as it's not in the DW 1.7 pack. Of the mods mentioned, it's also the least familier to me.
Blood Magic requires several diamonds and gold ingots to get going, placing it more mid-late game, especially if you want to progress quickly. (You'll want access to some form of self healing, be it healing potions or some other method, if you want to rush BM).
Thaumcraft is accessible slightly earlier than blood magic, the most expensive thing to find is 2 gold ingots. You'll also want a fair few ink sacks (or other black dye) for researching. Thaumcraft is one of those mods that you have to try to see if you'd like it. You can't get much in the mod without doing some research, which is pretty much a minigame in itself. This is not to everyone's tastes.
Witchery is likely the one of the 5 i'll cover that doesn't really lend itself to single player that well. Personally if it's in the pack i usually leave it to later game when it's tools are more useful. This is my opinion here. If you are on a multiplayer server that allows certain poppets you might want to get into this quicker than I usually would, if only to get you protection poppets against vodoo, or the other protection poppets. (also keep your bed safe,people can get taglocks from it.)
Personally the magic mod I tend to start earliest is Botania. Sure you need a couple of gold ingots to really get the mana system going, but other than that, it's all stone, wood and the magical flowers that are sure to be around. To get some toys you will need ender pearls or diamonds however.
I'm just going to list what i consider the perks of the mod that would likely be most useful early game. I could have easily missed something, especilly with Witchery and Blood Magic. I haven't included Ars Magica due it it not actually being in the pack.
Thaumcraft: Wand Focus: Equal Trade (Instant mass redecoration), Axe of the Stream (No more leaving a peice of tree up, although personally i prefer Ticon Lumber axes), Boots of the Traveller (speed boost, step assist and jump boost).
Botania: Manasteel Armor (Think iron armor that never takes damage as long as you have a mana tablet/band of mana with mana), Sojourner's sash (boots of the traveller, in the belt slot, I think they might stack.) Many other baubles and trinkets that are more situational. Rods of the Land and Sea (Makes dirt and water respectively from mana). Rod of the Skies (Sort of flight, but it's more of a huge upward leap)
Witchery: Sapling mutation? Seriously I don't know many huge benefits to having witchery started early. Although I'm not fond of needing to kill wolves to get certain things... (I love dogs
)
Blood Magic: Sigil of Air (Rudimentory Flight), Lava Crystal (Smelting for LP, although this isn't hugely useful until you have automated LP or a fairly large store of LP in my opinion), Sigil of the Green Grove (Fast Crop Growth)
Again this is my opinion, but I'd start with either Botania or Thaumcraft. Botania has a special mention if you are particularly interested in Blood Magic, having multiple methods of keeping yourself healthy while donating to your blood alter.