I don't know how I feel about helping with this, but here goes...
Number one: assume you're getting caught. Don't even try to be sneaky it's only going to make you look bad. I would expect IT to come down the day you plug it in. IT is probably worried about introducing security risks to the network, and rightfully so. Wireless access points are a pain to secure.
If you plan on using the Belkin N1 Vision for your WAP, its WiFi Protected Setup feature is vulnerable to attack, and very easily cracked by anyone with literacy, a keyboard, and interest. Turn that feature off. Unfortunately, plenty of routers are vulnerable even after shutting the feature off.
You also don't want to give access to the entire network over WiFi. I can't imagine your mobile devices need much access, and I'd hope you're not trying to access the email server from your mobile devices. As nice and tempting as it would be to have work email on your phone, stick with external access if you have it. You don't need to open vulnerabilities to the email server.
I'm not a security pro at all, but I imagine setting a firewall rule that blocks any and all traffic that isn't HTTP (port 80) might appease IT's imminent frustration. If you have access to any intranet sites (anything on the internal network that's run in a web browser. Does anyone use a 'program' in the office that's run in Internet Explorer?) then this rule won't be enough. You'd have to figure out a firewall rule to block all traffic that isn't port 80 and isn't headed to the Internet.
You'll also need to use WPA2 to secure the connection. Don't even bother with WEP it's, again, useless against anyone with half a brain, a Backtrack CD, and their older brother's laptop. Use the most absurdly long and complicated password possible. Think 63 (or 64? I'm not sure) characters of utterly random gibberish including special characters, numbers, and upper + lower case letters. Maybe use a generator. Now either sear that password in to your mind or write it down once and hand it to the guy who's going to take the fall for this debacle and have him keep it safe.
If you do anything less than this, the Men in Black have every right to neuralize you. They might even be mad anyway, I don't know how security paranoid your company is. The goal here is to not be the source of a massive security breach. They probably wouldn't set it up this secure, but they have the authority to make that decision. You do not.
Now to focus on not screwing up the network itself... I think NAT should protect you from IP address conflicts, but I'm not going to say for sure. Try to pick a range that IT wouldn't ever use anyway, just in case your router derps and turns off NAT for no reason (I've seen home routers do worse.) Try something in the 172.16.0.0 – 172.31.255.255 range. I've personally never run across a class B private network before, but it could happen.
As far as setup goes, the manual is your best friend. Try dynamic mode and see if you can connect, but I think this might disable NAT? Most of these protocols are meant to connect to an ISP. The real problem is finding a mode that works with the router but doesn't disable NAT. Someone with more knowledge could clear this up, but honestly I don't think they're interested in helping. It's going to be trial and error.
Personally I would forget about the WAN port and just plug in to the LAN port. Put your access point's clients straight on the network. This might seem counter intuitive for security, but the clients are getting access either way.
Which brings us to the DHCP server. If it's filtering clients by MAC address your company is at least somewhat concerned about security. You'll have to get dynamic/DHCP mode to work and use a MAC address accepted by the server. I recommend the NIC on the department head's computer. Copy that to the router and connect him directly to one of the LAN ports on the router. If you leave him on the network like normal you'll get a MAC address conflict.
If any of this doesn't make sense, Google it. If you can't handle that, you run the risk of really screwing something up. A colleague of mine once made a typo setting a static IP address and brought down payroll's SQL server. It was a dark, dark weekend. This could be you.
Your best course of action would probably be to continually bug your IT guys to get it done, and if after a few days you're still getting blown off, try plugging in your home router and see if you set off any alarms on their intrusion detection system. That'll get them going for sure, but probably not very happily. If you fail to set off any alarms, try opening up a packet sniffer on the network and then 'accidentally' create an IP address conflict with your own PC in an attempt to set off... anything that'll get their attention and make them pay attention to their frustrated users.
Have you tried bringing them coffee instead? I've written programs for a cup of coffee...