I do not think "shared" storage is practical for Adobe. There is no way to lock the files to prevent accidental simultaneous usage of the same files. This may not be the proper terminology but it sounds like you could create problems.
If you want to see what hardware is good look at our PC (only) Premiere Pro BenchMark PPBM5 web site and then realize that Mac's have major limitations on hardware configurations and the current Mac Pro best CPU the X 5675 originally became available in February 2011, and there are many newer faster better CPU's--but not from Apple
Points 1 & 2 - as Bill said, do NOT try and use shared storage for direct editing
Use your shared storage as a central file location, and copy the files to the computer's data drive for editing... and back to central storage when done... you may want to check into storage management software that has a check out/in feature, to be sure 2 people don't work on the same file at the same time
Based on past messages, no version of Premiere is designed to use a network for direct editing, only a computer's local hard drives
Not in a Network environment http://forums.adobe.com/thread/771151
-a work around, of sorts http://forums.adobe.com/thread/957523
-and not on a "domain" http://forums.adobe.com/thread/858977
Do make sure you have at least TWO hard drives in each editing station... three hard drives, or an internal RAID setup, may be needed... depending on what you edit
I dont see the shared storage as a problem, depending on the storage solution. If correctly set up, that is. You could even get a simple solution, which allows read-only access to your footage for an editing account, while keeping read-write access to an dedicated ingest workstation. Or something more higher end like that one: http://www.editshare.com/index.php?option=com_content&view=article&id=264:editshare-storag e-goes-into-overdrive&catid=43:press-releases&Itemid=184 or the one you mentioned. Haven't heard anything bad about Smalltree lately, on the contrary, but my experience with them is dated and I have really no idea how their support is today. EditShare, on the other hand, appears to have a decent support infrastructure. Had a nice chat with them recently.
SmallTree & EditShare (just as examples) also offer backup solutions for your SAN, which you WILL need. If you choose their solution, you're ready to go in a few moments. Otherwise you might want to look into local LTO drives for backup of projects & media. Having a backup on a harddrive is not really a backup at all, in my opinion. Just an easy way to keep your data available. The easiest way is to contact your local reseller and ask them.
Of course, you will need to set up your workstations properly as well, but thats not that difficult. Just let nobody tinker with the settings...
On the Mac side: that depends on what kind of footage you are shooting, or, more precisely, what data you acquire. SD, HD - and what flavour, 2k, 4k, 5k. Once you start using RED, for example, you might want to get either a MacPro with its meager performance nowadays, or an iMac, a Thunderbolt PCIe exteansion and a RED Rocket card as an intermediate solution until Apple finally releases a new MacPro.
Workstation storage is antoher thing. For CS6 - especially if you want to use AE as well - I'd recommend at least 16GB RAM for an iMac or 24GB for the MacPro (double that, RAM is not that expensive). Mind you, you can work with as little as 4 GB, but I wouldn't recommend that. You'll need a harddrive for OS & software, an external TimeMachine backup for that (a simple NAS like Synology or Drobo works perfectly well).
Then you'll need a RAID for Render & Cache (either using a dedicated RAID card, look at HighPoint for decent value/price, or a simple eSata solution - it is possible to use eSata on a Mac, just some cabling needed on a MacPro or a more tedious procedure for an iMac). Dont use Apple's Software RAID, since it's performance is... not there. And, perhaps, another RAID for your final files. Depending on the RAID solution, you'll either want SSD's or regular harddisks. An SSD RAID is really nice, but I suspect you won't need it. So, get a couple of decent hard drives for startes - preferable those you have in the SAN to make life easier later on - since you can switch later.
Graphics - now there is some room for improvement, on the Mac side. First, since Nvidia released it's latest driver for Mac, most of the later GeForce cards do work, albeit not the Kepler-based cards. I myself replaced one of the ATI5770 with an GTX570 and it works beautifully, easily on par with the Quadro 4000. That is, in PPro. AE is another beast. For PPro, you need horsepower, not large GPU Memory, while AE needs both and the GeForce's reference design is hardly above 1.2 GB in memory.