Ditto on the reliability of the Meridien systems. I’ve used two Symphony’s and owned an Avid Express Elite and have had no hardware trouble. They aren’t the fastest things around but they work with long timelines very well, which is one of the weaknesses of every other NLE I’ve ever touched.
As far as merging your newer Express Pro HD with the older Meridien hardware…I can’t comment with any experience but I would be doubtful that it would work. Don’t take my absolute word on that as I’ve never tried it, maybe someone else has and could post a more knowledgeable response on this.
Good Luck
John Watts