...I'm picking up a walkabout again ...
...should I be thinking about sending it to mesa to get it "updated"? What does mesa do to these older units to bring them up to date? Any opinions and info much appreciated! thanks.
If I'm not mistaken, the updates would include changing from two input jacks to one and an active/passive switch and adding a mini toggle switch on the back to bypass the effects loop. Both updates address the problematic switching jacks that seem to plague early versions of the Walkabout (mine included).
Price wise, you can check the Classifieds here on TalkBass as well as visiting: Guitar Center and searching for "walkabout". That will give you some upper end real world pricing on used Walkabouts.
Keep in mind that used gear purchased online/over the phone from GC can be returned to your local GC for full refund (except shipping charges which I've found to be very reasonable) within the standard 30 day Satisfaction Guarantee period if you don't like it. And, you may be able to negotiate a better price than what is shown online. I've had (too) many good experiences buying used gear from GC over the phone (and no bad ones).
Good luck!!!