I would definitely recommend camping within the circuit. Maison Blanche or Blue Nord would be my pick for atmosphere. Nothing quite like hanging out with a beer on Mad Friday, watching the party get started. I'd also suggest you watch the start near the Dunlop bridge and try and wake up at about 4-5am, head down to the track with a flask of coffee and watch the sun come up. Going to sleep with the sound of cars going round is a special experience. And you
will sleep - it's pretty exhausting as a spectator and lots of beer doth flow.
We usually head down very early on Friday and leave in our own time on Monday morning. Thursday is better, if you can do it, but leaving on Sunday evening feels very rushed. Couple of tips for the way down - watch our for gendarmes, avoid Rouen city centre (one massive traffic jam) and get yourself a Sanef toll tag. This year there might be trouble in Calais as well, so keep a close eye on that. Ferry is definitely the way to go - just for the ceremonial P&O fry-up and the sound of exotic machinery firing up on the car deck!
We used to organise ourselves but now book every year via Speed Chills because they don't charge a huge markup and have been really flexible in the past. We've also been with Thomas Cook Airtrack, which is glamping with a bunch of Lambo and Aston owners - quite pricey and probably a little isolated from the "real" Le Mans experience - but good craic, nevertheless.
Re. the weather, it can be very mixed but it's rare that's it's 100% sunny or 100% wet. Take waterproofs and suncream. If you camp, take a gazebo/shelter or you'll be couped up in your tent or car for hours. A portable radio and comfortable headphones is also a good idea, because most of the public commentary is in French. Fold out chairs are also an essential item. Pretty much everything you can buy down there though - we have a fairly well-rehearsed drill now of splitting into two groups; one to set up our pitches and the other to hit the Super U for beer/gas/steak.
You'll love it. I'll try and post some pics on here from previous years.