In my experience a very basic standalone VHF/UHF mod meter has limited value and probably isn't worth building in 2014. I know this isn't what you want to hear but unless you want to make something 'modern' and very accurate using (high speed) DSP/SDR technology then I see little value in making a basic and crude dinosaur that can be outperformed by a 35 year old classic like the Racal 9008M. These old Racal meters are available for very little money these days.
The classic commercial mod meters will also offer high quality demodulation with an AF output that can be fed to a scope or soundcard and this feature has been far more useful to me over the years than being able to accurately measure FM deviation or AM depth. The demod function in the Racal 9008M is very linear and can be used to measure the mod distortion performance of all but the very best sig gens.
In a few short years you will be able to buy wideband SDR based stuff from the far east on ebay for very little cost that will outperform everything mentioned so far on this thread