I'm not sure that moving to the 828mkii will be much of an improvement, especially if you need more than the two XLR IN's offered by the MOTU. I'm also not sure if the MOTU preamps/converters are any better. In this price range, I would guess that they simply sound different. You may want to compare specs, however, particularly if you have an older Mona. (What's the deal with Echo names? Mona, Gina, Layla? Are they old girlfriends?)
The other problem with "upgrading" to the 828mkii is the issue of reliability. I read nothing but horror stories when it come to the newer 828's. There are known issues with the front panel LCD meter system (it goes dim and eventually dies), multiple users report that their computers don't recognize the 828 on installation/startup, and there are conflicts with certain firewire cards (remember, the 828 is based on a firewire connection not a PCI card). There are also the usual problems with dropouts, noise, crackle, etc. It's hard to summarize all of the problems but you should check out the MOTU users group at unicornation.com. Take a close look at the Hardware Forum. And be aware that if you do by MOTU, they have terrible reputation when it comes to technical assistance.
If it were me, I would see if the Mona could be upgraded by bypassing the AD converters and preamps. This will probably increase you budget, but check the Mona specs to see if an external preamp can be inserted after the built-in pre. Or look at the possibility of setting up an external converter running into the Mona's digital IN.
