Those Viragos bring back memories! My dad used to own one, didn't have it long if I recall. Don't think he got on with it too well.
Look for the usual, suspension being in good condition, brake wear, general wear and tear. I tend to look around the welds around the frame, to see if its been bent from dropping it etc.
And as mentioned already fork seals
Have a look at things like the yamaha dragster 650, very comfortable for cruising around looks like a big bike but is actually quite light and feels smaller when you're on it than it looks from the outside.
I've ridden my dads many a time, not the quickest thing around but its different and pretty awesome. We've not had any problems with ours and we've had it for quite a long time now
http://www.ebay.co.uk/itm/YAMAHA-DRA...item3f2246da1f