Dither,wordlength,jitter?

There are two things you can do to find the best DAC: First you can measure the output of fixed frequency signals. There are a lot of these around (Stereophile etc). You will also need a scope for this. Without that your ears are the best tool. No amount of claims can be a substitute for a listening test.

Yes, all CD transports output 16-bit words. Not sure how much of an improvement a 24-bit DAC provides on 16-bit words _without upsampling_ (jury is still out on this). A good 16-bit DAC is still better than a bad 24-bit DAC. But the general consensus is that a 24-bit DAC with upsampling sounds better (on 16-bit data/music) than an equivalent 16-bit DAC without upsampling. Though not all 24-bit DAC's upsample. There are single-box players with resolution enhancement, but very few.

the dcs purcell upsampler is the product that started all the upsampling trend, although those who do not believe in these products liken the piece to oversamplers, they do increase the wordlength as well as the sampling frequency, it all becomes "technobabble" until you hear it. Then I think you will be a believer. Go audition one and see for yourself, take along both a great recording and a less than stellar recording that still gets your toes tapping. We look forward to your findings after you audition one. Personally, I will be getting one soon.