Aside from the costs of the wheel, you are going to have to be carefull when swapping them. Unless you move your rear sprocket you are also looking at a chain and front sprocket set too, and you'll need a second det of brake pads (I would advise some off-road compound - it's much better than the stock stuff, and a bit cheaper too).
You'll realy need to get rid of those footpegs too - they have no controll or grip in the muck. You can get a cheap pair from ebay for around �25 delivered, although they aren't the best quality, but they are very good value for money.
Hand guards ? do you have them ? if not you are going to be looking for some spare leavers, or some pivot levers (
www.racefx.co.uk).
On balance, for off road riding, why not just add a few more quid and pick up a rough 250/400 bike to ride the trails!
The other option is to invest in a tyre changing station at about �500, and swap the tyres as you like!