I haven't done any walking in europe (at least multi day) but have travelled reasonably extensively as well as done multi day trips in the UK.
Firstly if you are either sociable (make friends very easily, just walk into a pub and start chatting) or sure of your own company (nights can be a bit lonely) definately go for it!
Secondly, location, I've been very tempted to do some/all of the Pyrenees, which I personally think is a bit nicer than the Alps for walking, however at the moment I have the time but not the money (and am not too keen on spending weeks alone). There is actually a couple of routes that take you from the Bay of Biscay all the way to the Mediterranean via the pyrenees that usually take around 8 weeks.
Thirdly, equipment. You mention camping, which means equipment which, especially decent light stuff*, can cost quite a bit. You're looking at £100 on a bag (something in the region of 45-50L, i'd recommend something like the
Osprey Atmos 50, but you'll want to try one to see what is the best fit) and over £200 for a tent alone, sleeping bag will probably set you back around £70 minimum etc. You're also looking at other random bits and pieces like pen knives, stove/pot, thermal air bed (eg a thermarest) and a trowel (good luck trying to dig a hole most of the time though
) as well as maps, compas AND GPS (and i'd advise taking a second emergency compass, just a small thing that you can attach to your bag, whether you take a GPS or not, just in case you lose or break your other compass).
You may already have all this stuff so I'm probably telling you how to suck eggs but...
Either way once you get there it will be cheaper, you should be able to camp for free (wild camp) most nights depending on the area. With wild camping you need to think about three things really (again not sure how much experience you have); toilet, which is where the trowl comes in, if you can't find a village and proper toilet; cleaning, either be smelly for a while, until you stay in a hostel for a night, or wash in streams/rivers; water, most important really and generally you should be able to stock up with a few Litres every day or two in local villages.
I'd recommend you get a lightweight tripod, you'll miss it if you don't have it (I use a Giottos
GB1060, small in height but cheap and light). I've actually put a list together of what I take with me if you're interested, it includes some weights as well. As you suggested around 3-4 batteries and probably 16GB of card space with maybe a backup. What you should be able to do if you don't want to carry around an external HDD or netbook type thing is burn whole cards to DVD at internet cafes (reasonably common in villages in Spain, i'm guessing the same in the Alps) and send them home**
Definately go for it, if you are like I mentioned earlier it would be a trip of a life time and something to remember! If you're not adverse to a bit of hard work and travelling a bit further there are various trail organisations in the US that provide food and lodging (well normally tents) for help maintaining trails (such as the
Appalachian trail), something i'm also contemplating doing this summer.
Actually, darn you, you have me thinking about doing this again! It's still a bit wet (only have tarp and bivvy) and cold to carry on with my trails around the SWCP but Spain sounds tempting...
*which you will want, you won't want to be carrying more than around 15kg max, which isn't easy with some camera equipment.
**The usual suggestion is to burn each card to two DVD's, send one home at the time and keep the other until you get confirmation of the othe getting home safely, then send the second DVD the next time you burn a new card to DVD, along with one copy of that new DVD. That way there is always one copy of your photos in a safe location (either on you or at home) and you don't have to worry about carrying a whole trips worth of photos with you.