the sigma 18-200, cost around 250 pound correct? what do you intend to use it for? there may be a lens more suited to your needs. if you tell us what your to be using it for, we can more easily say if it will be sutiable.
with lenses, a prime offers the very best quality optics and thus, best image quality, the introduction of zoom lowers the quality. the wider the range the lens is, the harder, and more expensive therefore, the lens is going to be.
considering how much you've got to spend (that last lens is about 500-600 pounds) id recomend whatever the nikon version of an L (luxury) lens is.
dust on the sensor is not as big a problem as some folks seem to say it is, ive changed lenses a hundred times and mine is fine.
It will be better for covering the 70-200mm range than an 18-200. However it won't be any good for landscapes if thats a requirement this lens MUST fullfil.
Changing lenses isn't a problem. I do it in all conditions and as long as you excercise some basic common sense you're fine.
put new lens face down (with cap on!) and loosen the rear cap on this lens. point the camera down (make sure lens cap is on old lens too) drop out old lens. put camera over new lens and transfer rear cap across to old lens. drop camera onto new lens
done
its important to keep the camera pointed down, its much less likley to get stuff in it.
as to your choice of lens - since you already have something at the 18mm range, the 70-200 would be much better, the quality of the shots you get will be higher, a 18-70 and 70-200 are a good pair of lenses, obviously covering most focal lenths you would want.
ALSO, having to change lens to go from wide to long may force you to think about what your doing, this is one of the arguments for a prime too, forces you to think about composition and not just throw it together in the view finder.
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