You wont find a 20mm axle on any XC forks, thats on more DJ or FR/DH. I dont think you would have to convert to disc. That depends on your wheel not your fork, unless it doesn't have a V-brake mount. Now, you could buy it now,
http://cgi.ebay.ca/Rock-Shox-SID-Rac...QQcmdZViewItem
The SID is the best XC fork rock shox makes and that version is the SID race which is a very good fork. You'd be saving around 200$ for this fork.