Advice from someone who has done a lot of traveling with a laptop;
It sounds from your post that you are going to be carrying your stuff yourself. My advice is to make it as light as possible, preferably with wheels. The SKB case mentioned above is neat, but you are going to absolutely hate carrying that thing around Europe (just picture that for a minute... those cases are pretty big, unbalanced and have crappy little handles, I had one). Also that SKB case will not protect your laptop on a plane, it's just not made for that.
I think getting a "real case" for a laptop is just a bad idea, it will be heavy, expensive to fly, and a huge, huge pain in your *** at 5 am when you have had 3 hours sleep.
I usually do either of these two things in a "carry your own stuff" fly-date situation, maybe you can come up with a variation that works for you.
1. A really good, well padded backpack designed for laptops, like a brenthaven or something, which you keep by your side on the bus, plane, van etc... you have to keep it with you and can't pack it in a cargo area.
2. A rolling hardshell carry-on suitcase (a good one like samsonite, not those cheesy aluminum things) and you set it up so it's padded from shocks inside, maybe in a good padded sleeve, with your interface, cables etc. If ya really want to do it right and make your life a little easier on the road get some foam, an electric carving knife and set up the laptop and interface inside the rolling case. You just open the case, plug in your guitar, hand the sound people your output cables and play. Especially nice for festivals where you sometimes have very little set up time. Make a strap to hold the lid of the case up at an angle.
This rig you can pack with the other suitcases or light gear... it's still on the borderline of not quite safe in bays full of heavy gear.
Even if you are not carrying your own gear and have cartage, I would carry the laptop and interface with you like above and put the rest of your gear in a medium weight case. The heavier the case, the more abuse it gets.
Rain... can't help ya there, most stages are covered, there's just a little bit of electric gear on them already, maybe have a short handled umbrella handy in case of sideways rain.
yada.
-R
