Well as Spa will not be hosting a F1 GP next year, I imagine tickets may be a bit hard to come by, but if you do go I imagine the crowds will not be a problem GT's are visiting Spa though, so this maybe an option for you
IMO dont bother with the farcical F1 races and spend your money on a trip to on one of the "oldtimer" events at Brno in the Chezch rep or at the nurburgring!