Snow Leopard requires only that you have an Intel processor, which you do. No processor upgrade is necessary. (None is possible, either).
Unlike Neville, I don't recommend using an external hard drive as your primary boot drive with a notebook computer. Doing so would sharply limit the portability of your MBP, since you'd always have to carry the external drive around with you. Anyone who carries a notebook computer is far better off upgrading its internal drive to hold as much as s/he needs to have with him/her while moving around. But if portability isn't an issue for you because your computer always stays in the same place, then Neville's suggestion is a good one. The internal hard drive in your MBP is not considered by Apple to be a user-replaceable part, and unless you're a reasonably capable do-it-yourselfer it may not be something you'll want to tackle. DIY instructions are here:
http://www.ifixit.com/Guide/Repair/Installing-MacBook-Pro-15-Inch-Model-A1150-Ha rd-Drive-Replacement/486/1
But as others have already said, you don't need to install all (or any) of the Box Set applications if there are some that wouldn't be useful to you. You may be able to get by with your present hard drive. Just be sure to keep 10-15% of it available as free space at all times.