Not hard at all. Actually I took a few runs at it the first time for my .300 because I couldn't believe it was really that easy. You take the ejector plunger out of the bolt, close the action on a go gauge, screw the barrel down onto the gauge then tighten the barrel nut. After that you check with the no go gauge. If the bolt closes, loosen it all up and try again. If the bolt doesn't close you're good to go. I like to check a piece of sized brass just to make myself feel better.
The beauty is, headspace can be a trial and error (before firing of course!). If you get it wrong and the bolt closes on the no go then all you have to do is loosen things up and try again, no trips to a machine shop for a lathe or tedious reamer work.
you can rent headspace gauges here for a reasonable price http://www.reamerrentals.com/Product...tCode=300WSM.g
Here's the wrench you need. If you shop around you might find a better price.
Wheeler Engineering Barrel Nut Wrench Savage 10, 110 - MidwayUSA
I'd go for a long action so that you could load long bullets and still use the magazine. I don't know for sure, but I would bet that the magazine box for either the 7mm mag or .300 win could work for the WSM with little or no work.