To your inquiries:
We are a Digium partner and have been setting up Digium Switchvox systems in environments similar to yours for over 4 years and have seen some major growth in both the back end and "front end" (read: phones) from Digium.
What you have read recently regarding support is really a new-ish issue and Digium is committed and has definitely made improvements from the horror show that was their support line just a few months back. That being said, up until that point (for the majority of the last three + years), their support has been very good.
Also, documentation is rough, but each page on the admin GUI and web user GUI has a ? icon with inline support, but it a bit difficult to pull out what you need from them. I.E. the help sections only really help if you KNOW what you're looking for / at.
Also, with all that being said, your local Digium Partner(s) should be able to supplement and/or surpass Digium's own support in timeliness and effectiveness. Most of our clients, even if they buy Gold support (but most buy Silver), come directly to us as we have a dedicated staff that's "in the trenches". Therefore, we can provide better real-world assistance, versus Digium's scripted support lines.
Lastly, each 355 will support up to 99 concurrent calls. The more people that are actively using the Switchboard and/or the Admin/User portals will certainly also take hardware resources from the box, so, where possible (i.e., remote sites) you may choose to run them on their own boxes and PEER the systems together, rather than running one or two "central office" boxes, if that makes sense.