There is no single "right" way to go. It's quite possible that there are several solutions that would work for you.
One important option that you might not have considered is to use a PC running Linux (or FreeBSD/OpenBSD), if your organization has any experience in that area. One major bonus is on pricing: with this approach you would not be tied to the pricing whims of your vendor and consultant. And you could add VPN capabilities if/when you need them for no extra cost.
Basic routing/NAT is trivial, DHCP, firewalling, load balancing are all very straightforward and much instructional material and help can be found for free here and elsewhere on the net (there are plenty of consultants you can pay as well if you want more formal assistance, myself included).
Bandwidth management and content filtering are necessarily more complicated topics depending on what kind of rules you want to implement, but there are solutions available in the free software world for these problems; indeed, many of the commercial products are simply repackaged versions of the same software.
If your organisation does not have any experience with Linux or the BSDs, it might not make sense to start just for this networking project; however, there are organizations that have done exactly this with great success. Or it might even be the beginning of a new IT strategy for you. I have known several organisations that have started with networking and then moved towards open source software in other areas.
Update/upgrade can be argued as feature in either direction; it's likely that commercial products will have a simple (but comparatively expensive!) 'update everything' button, on the other hand, if they are merely repackaging free software, you might be better off with the latest release of the source.
These are many of the same factors that affect any buy vs. build, commercial vs. open source decision.
The point I'm trying to make is that a rigourous analysis that correctly determines the most cost effective solution for your needs is probably far more trouble than its worth; what you need to do is find something that will take care of your requirements at a cost (time/money/effort) that you can afford.