Ok so here's my idea.
First off you want a pretty solid network. If there's a group of you sending mass emails your gonna need some serious bandwidth. I would suggest looking for multiple cable providers and maybe even 3g on top if you want the extra bandwidth
You would want to have about 100mbps ofbandwidth to put out some serious emails however it depends on how much you send.
I'm guessing you have some nice proxies though? If you do, I would host the emailing apps externally. Pick up some atom servers from interserver.net and then use the proxies. You should know that cable usually has a static ip so your out of luck if you don't have proxies.
Now for you local network I highly recommend you get a 1gbps switch alongside CAT6 cable. This will definetly help LAN speed locally when transferring data from one computer to another.
Now you should have some sort of server which can be accessed via ethernet. I would look into a RAID box or otherwise build a cheap Linux box up with a raid card and put FreeNAS on if you know what your doing. Always go for enterprise drives, since your using this a business better to be safe then sorry.
For accessing all your computers, I would get either a KVM switch like Keedev said or go with Remote Desktop if you have a 1gbps LAN network.
Since your using laptops as well, it would be a good idea to invest in a Wireless N router. I would go for a Airport Extreme(It's PC friendly and still one of the best routers around atm) or you can go with a Linksys WRT610N.
So the 1gbps switch and the CAT6 cable would be anywhere from $150-$1000 depending on how much you want to spend.
The raid box would be around $1000 if you get a raidbox or a linux NAS with enterprise HD's.
The KVM switch would be around $200 for a decent one.
And the Airport is $179.
So you looking around $2500 - $4000 for the whole setup
