I think Pieter's covered most of it and I can't be bothered to quote!
As Pieter says, it's a fair walk through the rear (Karting) access to the circuit - I think it might be more than half a mile. It is pretty flat though which is a novelty for this area! Some of the guys on the campsite takes bikes to go to and from the circuit.
If this works and you find Stavelot on the map, look to the right
for the Masta Kink and the campsite is just about where the "M" of "Masta" is (it's on the old circuit). You would then walk up the green road towards the chequered flag, the red line shows the entrance to the circuit and the wiggly road is the track into the rear of the circuit - it looks like around 4km including the wiggles!
http://www.viamichelin.co.uk/viamichelin/gbr/dyn/controller/mapPerformPage?strCountry=311&strAddress=&strMerged=stavelot&x=0&y=0You can easily drive up to the parking area at Les Combes though (2 minutes from the campsite) - I don't know what the cost of parking is (they let us park for free with our media pass), probably 5 dingbats or so. This gives you a very short walk to Les Combes and Malmedy corners and you can then walk down towards Eau Rouge (corner not campsite) etc. or the other way towards Rivage and Pouhon.
If there is an option to get parking in the circuit (it will be in the carparks opposite the endurance (not F1) pit lane) then I'd go for it, this would give you a good base to walk around the paddock areas, up Eau Rouge and up to La Source and the area previously known as the Bus Stop etc. and then if you want to have a drive up to Les Combes on the Sunday it gives you some alternative viewing areas.
Viewing spots - For me there is nothing that beats sitting in the trees above the Bus Stop and Blanchimont in the dark seeing GT cars whizzing underneath you.
Eau Rouge has to be seen to be believed and the back of the circuit Les Combes, Malmedy etc. is beautiful and you can look across and just about see the action through Blanchimont as they are at the same height!
Circuit Camping - I think there was a discussion about this last year, there is camping at Les Combes on the side of a fairly steep hill. The info last year was that there are no facilities here at all, no toilets or anything. I think it's quite cheap and I doubt you have to book. It's a bit hardcore for me though!
The Eau Rouge campsite has a bar on it which is quite lively and has decent toilets and showers. BBQ'ing is fine (although the bloke running the kids archery complained that KK smoked him out last year!), they have some half oil drums on the overflow campsite and it's OK to have fires in these.
Don't know about fuel prices - we run diesel and I'm not sure that there is much difference to France
Other things Spa town is worth a bit of a wander round, treat yourself to lunch or dinner in L'Auberge, it's lovely! In the old days of running races for the BRDC we used to stay there. There is also a funicular railway in Spa that goes up to a Therme (thermal baths - can't do the accent!).
Around the rest of the area, there are loads of little restaurants that serve good, reasonably priced food. Stavelot is worth a wander round, the whole area has a lot of WWI and WWII history, there are lots of memorials around the whole Spa area - too many of them for civilians. At the river at the bottom of Stavelot there is an American armoured car that commemorates a battle at the crossing of the river. If that kind of thing floats your boat it's worth doing some research before you leave as there are lots of significant sites around the area.
At Coo there is a cable chair lift thingie that takes you up to the top of a big hill (and brings you back down again!).
We think we're leaving home on the Saturday ahead of the race, stopping off in Reims/Eppernay for Saturday night/Sunday night and then arriving at Spa on the Monday.
Sign posting Niether the area or the crowds are big enough for there to be a problem.
Supermarkets There is a Lidl close to the campsite in Stavelot and another small supermarket that might be a Super U. Neither are big but have the basics. We also go to the Carrefour at Malmedy and they have a good supply of Jupiler beer! There is a bread van that comes to the campsite in the morning, you just order bread/croissants the night before and then turn up and pay when the man turns up around 8ish.
Can't think of much else!