The Vengeance base is what I'd go for over the stock WGP, simply because it comes with a better looking body, the System X front pneumatics kit, and the System X barrel, all of which are better than the stock. If you buy second hand you get more for your money, but if you're more comfortable buying new, go for it. The last thing you want is to get to the field and have your marker screw up, and you not know how to fix it. Regardless of what 'cocker you buy, you should posses a basic knowledge of it's workings.
www.moodypaintball.com/vantrepes is a good 'cocker resource, give that a look before you go out to the field. Suggestions of where to buy used? Why, right here at p8ntballer of course! Go to "forums" and scroll down to "Classifieds".
www.warpig.com has good classifieds as well. Good luck.