Hi,
No experience with Motu or UA interfaces.
I have been using a Focusrite Liquid 56 with both Sonar and Presonus Studio One and it is rock solid. The the Liquid 56 has 26 inputs available, but 16 are via ADAT I/O so not sure if that will help your input needs with the synths.
I recently purchased the StudioLive 32 board you mentioned and I am impressed with it. I mainly use Studio One and once they get the DAW functionality incorporated later this year it should be a great mixer/controller for Studio One as well as an audio interface for tracking.
Note - you will not be able to cascade the new boards as you can the older StudioLive boards (and it is a different design, so it is not compatible with the older StudioLive boards). Also, the new board will only support 44.1kHz and 48kHz sample rates. My understanding is that the new board will eventually support Mackie protocol, but not until after they get the Studio One support incorporated.
Friendly suggestion would be to contact Presonus Tech Support for your older Studio Live, they are really good people and they can sort out the driver issues better than you pulling out your hair. Also, if it comes to it, replacing an older PC with a newer one, may be both cheaper than a new board, and may make your setup work smoother - just saying.
Good luck and take care,
h