Those WWE Brawl Fightsticks are awesome if you can pick them up for $20 to $40. They are by far the cheapest alternative to the quality of a TE. Don't worry about modding them until you get to the point where the stock buttons and stick just won't do it for you anymore. This usually happens with about two months of intense playing or maybe 6 months of casual playing.
I have both the SE and TE verisons of the Fightstick, and for me, that was how long the buttons / stick lasted in the SE before I had to replace them with the Sanwa buttons.
Also, note that if you play Skullgirls on PC only, and don't care about console, BOTH the PS3 and 360 versions of the WWE stick will work on the PC (well, the later versions of the stick anyway). I've personally tested this with my friend's WWE stick, though he got his pretty late.
I see Amazon has them for $50 (http://www.amazon.com/PlayStation-WWE-Stars-Brawl-Stick-3/dp/B004M25NNQ/ref=sr_1_2?ie=UTF8&qid=1378570031&sr=8-2&keywords=wwe stick).... that's a little high, so if you can find them slightly cheaper, I'd go with that. Otherwise, maybe this will work for you?
The buttons should have "sanwa" written on them between the contacts if he lets you open the case. As long as everything else works I'd call that a steal.
No, not all PS3 fight sticks will work for PC.
To answer your questions....
1A) MadCatz Fightsticks Tournament Edition are the de-facto standard for the fighting game community. They were created several years ago by MadCatz with a lot of input from some prominent fighting game community members (MarkMan and S.Kill iirc). Thus, they were created by fighters for fighters. The previous king of fight sticks I believe was the Hori HRAPs before MadCatz showed up.
1B) The WWE model is the cheapest incarnation of the MadCatz fightsticks. WWE costs 20-40, the standard edition (SE) costs about 60-80, and the tournament editions (TE) run 100 to 120. For someone who is just starting out, you would want the absolute cheapest version to see if you actually like it before investing more time / money into it later. I believe the only difference between the WWE version and the SE version is that the WWE version has some ugly art in the front of the stick and ugly button colors, which is ignorable and easily replacable.
2) If you're not a fan of Sanwa, you can try Semitsu. They also make high quality arcade sticks and buttons. Other than these two, I don't know of any other high quality sticks / buttons you can throw onto a MadCatz and still have it be good
3) To give you perspective, this is exactly one of the sticks I own (Xbox360 version). I paid $70 for the stick brand new, and after a few months of use, bought button replacements. I only replaced six of them so that costed me around $20 to $30. Amazon has these buttons for $18 now apparently (http://www.amazon.com/Green-Sanwa-Push-Buttons-OBSF-30-G/dp/B003D8897M/ref=sr_1_1?ie=UTF8&qid=1378578087&sr=8-1&keywords=sanwa buttons). The Sanwa stick itself costs $25 brand new. But remember, I had brand new parts, bought it when it first came out, and you're buying just now used, so I dunno what your valuation for that would be.
Bottom line:
Buy a WWE Fightstick, and pay $40 brand new but you may or may not have to replace parts later which can cost you $25 for the stick and $20 for the buttons
Buy the Craigslist stick and pay $50 but no guarantees the guy isn't lying about the parts or stick, and you don't know how much abuse the stick has gone through with the previous owner.
For the record, people, I did buy that Street Fighter IV stick off of Craigslist, but it's a PS3 model and does not work with my PC, unfortunately. I still got a steal though, since it came with a Sanwa stick and 6 Sanwa buttons for only $50. I'm pretty sure that I'm going to buy a used 360 SF4 stick, swap the good buttons into it, and sell the PS4 version on eBay with the Mad Catz buttons.
All is not lost yet; if your PS3 fight stick doesn't work just from plugging it into the computer, I believe it may be due to the USB chipset. There's third party drivers you can install to try to get it to work the stick. I haven't done this myself, since my experience has been with the later models of the FightSticks, but I've heard some people have gotten it to work that way.
Read through this thread from SRK and see if it helps you any:
http://forums.shoryuken.com/discussion/174237/ps3-fight-stick-te-won-t-work-on-my-pc
Oh, and please post your results here for future users with the same problem :)
Edit: Did you say SIX Sanwa buttons, or EIGHT? There's eight buttons on a typical MadCatz Fight Stick.... I only replaced 6 of the 8 on mine because I don't use the last two, but if the Craigslist guy says they're ALL Sanwa buttons, you should have eight Sanwa buttons.