Doesn't look like a difficult seal to fix. I believe that the slave pushes out when you pull the clutch....if so, When you remove the slave, hold it away from your bike, push in the slave manually (if you don't you can ruin the slave...did this on a different bike ), then pull your clutch lever. The slave should push out without ejecting fluid. If so your slave is likely good.
As far as buying used...Its not my favorite thing to do but I know a lot of guys who have bought 10-20 year old used slaves and they've worked great. I figure on a 3-4 yr old raider that's been parted out due to a wreck you should be ok. That said if you have the bank go new.