OK, points in order:
1) Preparation: Pack as light as humanly possible. I can do two weeks with an Oxford tailpack, one humpback pannier and a tankbag for maps and electronics (the other pannier had camping gear in). Don't take a rucksack - you'll be sick of carrying the weight on your body within the first few hours and your back will be sweaty as hell. Get a tailpack. And this point bears repeating: pack light.
2) Routes and motorways: You can do Calais to Barcelona in two days, but if you think you'll do it without using motorways you're seriously mistaken. It's 840 miles, that's 13 hours' riding according to Google Maps and that doesn't involve fuel stops, comfort breaks and lunch, etc. I did Hoek van Holland to Brno in two days last year and we did motorway pretty much the whole way, a chunk of it with no speed limits. We were knackered by the time we got there. we were also off the boat in Holland by 8am on the first day, having done an overnight crossing. If you must do it that way, go over the night before and stay at a cheap hotel in Calais, so you can get an early start without checkin, transit and time differences eating into your day from the very start.
Average speeds matter. You might think you can cruise at 80 plus, but every minute you're not moving your speed is coming down. Your average speed will end up between 50 and 70 on motorways, between 30 and 50 on other roads. Seriously. While you can reasonably comfortably do 450-500 miles a day on motorways, I've found as little as 250 miles to be waaay too much if using interesting wiggly roads all the way. And you don't want a mad 150 mile dash for the hotel before sunset at the end of each day.
3) See above. 200 miles a day is plenty if you're on the twisty stuff. 500 miles is *more* than enough if you're on motorways. that's 10 hours of solid riding at an average of 50mph. 1000 miles is do-able if you absolutely have to get home and you're running late, but you'll be riding for 20 hours and you won't want to get out of bed for a fortnight.
4) Work out how far you're likely to get each day, look at the rough area, hit booking.com and find somewhere in the right place. You can generally cancel with no charge if you find somewhere better in the meantime. Camping is an optoin, but it means carrying more stuff and you probably don't want to do that. I lugged a tent, sleeping bag, thermarest and whatnot to the far side of the Czech Republic and back just so I could spend two nights at Gedinne at the very end of the return leg. Getting that on and off the bike every day was more than a bit tedious.
5) Tolls will add up. Fuel will add up. Accommodation will add up. Do get euro breakdown cover, you don't want to be stuck 1000 miles from home with no cheap and easy way of getting recovered.
Be realistic, and assume you'll get nicked at least once. I always carry a float of a couple of hundred euros just in case.
6) Nah, road trips on two wheels are just about the best experience you'll ever have. Go for it.
I'm tentatively planning to be riding down to the Pyrenées and back in mid/late June, I can bung you my draft route if you want to see the kind of stuff I'm planning. The two big trips I did last year are blogged at
http://eldaifo.blogspot.com/, but I go to France and Belgium a few times a year for weekend raids as well. Once you've got the bug, it's hard to shake.
Have fun!