James,

For what its worth, I recently did this same job with a borrowed indicator (digital Mitutoyo). In the process I discovered a couple of things that you might want to keep in mind while looking at your options.

1) While you are only measuring a 2 mm detent at the valve spring, the body of the dial needs to be several inches above the valve spring to clear the valve rockers and cam. So, make sure you have a "probe" that is at least ~ 6 inches long.
2) The set up I borrowed had a very strong rare earth magnetic base. The nearest flat surface that you would logically want to attach it to, however, is on top of the intake manifold which is aluminum . So, if you are planning on a magnetic base, make sure you have enough reach with the support arms.
3) I mentioned to one of the physicists in the lab next door that I was surprised at how inexpensive dial indicators were. He stressed that if I were looking to buy a new one I should go with a digital unit, as it is much less expensive to manufacture a high quality digital instument than a an analog one (with a traditional needle guage).

I would also guess that a 0.5" measuring range should be more than enough (I can't think of when you would need to measure a difference greater than that). More important is the number of significant figures, the more the better.