If you're looking to use the Meridian as a transport, and then improve it's DAC by adding something external -- and I'm sure folks will offer different opinions -- but you might find it pleasantly difficult to improve on the Meridian's internal DAC cheaply. Perhaps biased, but I think it's a pretty fine DAC as it is. Certainly, you can make it sound different, though -- what exactly are you after? Put differently, if you're looking to continue spinning physical CDs, if it were up to me (which it certainly isn't), I'd stick with the Meridian. You could spend quite a bit of coin chasing an external DAC for the Meridian and -- among interconnects, multiplying power sources, synergy challenges, and all manner of other variables -- very easily end up not doing any better.
If you're looking to squeeze more out of the Meridian generally, I found that mine (yes, have one) was very sensitive to isolation. A very naturally smooth and organic CDP -- definitely with the potential to be too much so. Proper isolation made a world of difference, enabling it to preserve all of the organic warmth I fell for, while at the same time significantly improving the snap and engaging factor. Ended up with a Neuance shelf under mine, but more options than worth the trouble of counting. You mileage will certainly vary, but my experience for what it's worth.
If, however, you're ultimately looking for a piece of gear you could run any sort of computer-based transport into, then the Meridian definitely doesn't fit the bill. (Really wish it did have a digital in, I'd love to be able to try out the Meridian internal DAC with the computer as transport). If you think that may be in your future, then definitely worth looking for external DACs. I walked just that path, starting with a MHDT Havana (which is quite nice, but definitely not better than, or even equal to, the Meridian to my ears), and currently using an Ayre QB-9 (neither of which, admittedly, ever got plugged into the Meridian). In short, if you picked up the Meridian for ~$1k, consider it a bargain and I suspect you will have a a real challenge improving on it's sound with an external DAC for less than what you paid for it -- which kinda defeats the point. Me, ended up having to spend almost three times that for a comparable or better sound from an external DAC. If you're committed to walking that path and/or converting to a computer-based server/transport, then definitely worth it (and required). But if that's not the direction you are headed, wouldn't bother. Again, just my opinion -- having been through the same process -- for what it's worth.