If you are buying the system separate, I would not recommend the Velinion setup. It's just too expensive for what it is. If you are looking for something out of a local shop, you'll pretty much have to just go see what they've got. If you are looking to order online, then hobbypartz.com isn't a bad place to order. This link
will take you straight to the motor/esc/programming card combos. 13.5 and 17.5 combos are both $120.
Honestly any setup should work fine, though if it's for younger ones I wouldn't go too crazy, no lower than 13.5T. Also, pay attention to the Turn rating, a 1/10 motor may not be a full 540 size motor even though it appears to be. Whole numbers (13T or 17T) are usually a smaller armature, and can usually be told visually by deeper fins on the motor can. Half numbers (13.5T or 17.5T) are true 540 size motors, and usually have smooth cans or very shallow fins.
Either way, I would definitely recommend Lipo. Hell, you may wanna do that first, since that will actually make the stock Titan a fair bit quicker on it's own, though it will also let you burn the Titan up quicker as well.