I am in the process of a 'slow & gradual' restoration of my WC51 and have found it to be 'mostly' a great deal of fun and enjoyment!! (not joking......really!!)
I support both Gordon and Goran in their views that they are really easy bits of kit to lookafter and restore - almost a large airfix kit if you keep a record of what bolt goes where when you take them off!!
In terms of looking out for things... there's always the usual mechanical and electrical stuff to consider as you would any vehicle - eg;- whining gear box, engine knocking, intermittent electrics, etc.... however I would strongly recommed you try and find one that has been in Norway as much of their stuff was dry stored in the post-war years and had very little use. As an example mine came from Norway and has very little 'proper' rust on it and is being sorted by a little sanding to bring to bare metal to clean up before painting.. If you find one in France for example I have seen them very rusty in the cargo compartments of the rear bed or the cap floor - which are not very big jobs, but they are more work!!!
Also.... brakes.... you will probably find that if the truck has been sitting for some time the brakes need replacing or at least 'work'. As the guys have already said, parts are very easy to get hold of and there is a cracking support network of people around the world on different forums, including HMVF who are willing to help / advise / etc...
I suppose, in the end, it depends upon what you want to spend and what condition of vehicle you are willing to go for. You could easily spend around £3k on a barn wreck needing almost complete renovation upto nearly £12k for an almost perfect example with new canvas, correct bar-grip tyres and other 'stuff' included....
Have fun looking....and enjoy...
Gareth