I'm not a high level tech guy. I maintain our server at work. We use a raid 5 array at the office. That required at least 3 drives. We also have a gigabit network. I can't give you "numbers"... But I think that switching to a gigabit network will be your biggest improvement. You're increasing your throughput to the server by roughly 10x... Keep in mind the server probably only has 1 NIC [network interface card] configured, if it even has anymore. So the server can't run any faster than 1x gigabit connection. If it has multiple NIC's you can have those configured to run as well which will help your throughput. I'm not sure how that's done. But I know it can be.
So if 10 users hit sync at the same time, it's still going to think about it. You still will likely hit a slowdown.
My personal opinion on SSD. They are great... But not for servers (yet). The high costs, and greater than magnetic drive failure rate is... Well... Scary. Since you only mentioned only using 1 drive I assume your not using any kind of array, or you're considering just adding this as an additional drive for extra storage. Hardware failure is a real problem, you'd want to make sure you have a good backup daily. Even semi-daily with that many people working on it.
SSD's on workstations are great... Of course you probably don't need more than a 256gb drive for that. Nothing beats a reboot time of ~30s.
We have an HP ProCurve for our switch. It works really well. I plugged it in 7 years ago, and haven't touched it since. Buy good quality hardware. Don't get a cheap piece of crap because it wont have the throughput you really desire on it, or it will cause problems down the line.
I highly recommend talking to an IT professional in your area - find one with architectural office experience. They do exist, and it is helpful.
If you have any other questions I'll be glad to help. Good luck.