Confused is good
Where are you going? If in Europe, then I can't stress the need for an UWA enough. I *wish* I had my Sigma 8-16 when I went to the Vatican, but alas, I didn't own one at the time. 18 was not even close to wide enough. Also, I find the UWA's great for the outdoors too. You can super wide landscapes without having to stitch panoramas together.
Regarding what lens specifically, I don't have any recommendations because I've never used them. I went straight to the 8-16 (after having the Pentax DA 12-24 for a bit), and I have no regrets. But it is a bit pricey...
Like I said, the 10-24 f/3.5 is not highly regarded. Yes it is faster than the f/4-5.6, but everything I've read is that the performance on the f/3.5 is not as good, and it's more expensive.
If you are going to buy used, I would recommend just snagging up the first one in good condition you can find. eBay, KEH.com, Amazon, PF Marketplace, etc are good places to look for used gear. Sigma also has a used outlet store that sells refurbished gear (never be afraid of refurbished - usually they are brand new for significantly cheaper), but I just checked and they don't have the wide angle zooms for Pentax.
Best of luck
-Heie