I think the reason nobody has responded, is because this is an awfully big kettle of worms. You're talking about a number of issues, each of which is a discussion in and of itself.
I'll try to address a few of the concepts that might help.
First things first, I'd really kind of recommend you to split your networks up, and especially in terms of the public internet access. What I'd do if it was me, is buy a 2nd internet connection for your customers to use. Get a cheapo DSL or cable internet. Doesn't have to be expensive or fancy. Just something that works and has a little bandwidth. Put your d-link router on that network.
Then I'd put your business stuff behind the firewall with the tpac connection. If you bought an upgraded license for the ASA, you could run a DMZ. A DMZ is basically a place you put stuff that needs to be accessed by other people and is hence insecure, but in such a place if it is compromised, they can't take over your internal network.
One thing you should really look at is using your 3550 switch to implement VLANs. Make several subnets in your building, like vlan 100 customers, vlan 200 business, vlan 300 servers, vlan 400 wireless, whatever. Then lock it down with access lists, so that people on the business network can get to the customers network, and customers can get only to your file server, and wireless users can only get out to the internet, and so forth.
Use the 3550 with vlans to segment your network, then you can attach a dumb gigabit switch to that or whatever if you want to copy files faster between your computers. The 3550 may bottleneck you to 100mb connections, if the server is on one vlan and the PC is on another. This also gets into issues of how to serve DHCP on the networks if needed, enabling layer 3 routing on the 3550, etc.
You might have better luck to focus on only one thing you want to accomplish, post a picture again, and get some advice that way. Once you work that part out, post again with the next enhancement and so forth.
In my opinion, the fastest way to get out of your mess, is 1) Move downloading users to another internet feed, 2) Implement vlans to segregate users and traffic.
You could combine both solutions, if the d-link router would let you add routes. Basically, in my ideal world, you'd buy a DSL line, put the d-link router on it and set up a customer access network. Put that on vlan 100, say, on the 3550 switch. Your d-link router could be 192.168.1.1/24, and your 3550 switch can be 192.168.1.2/24. You put a route on the d-link that says if someone wants to get to 192.168.2.0/24 (your server network) or 192.168.3.0 (your business network) then go to 192.168.1.2. Add similar routes to your ASA firewall pointing back to the 192.168.1.0/24 network via 192.168.1.1, and you could have a two headed network. Most of your internal stuff could use telepacific, while sending the other users out via the DSL.
Hope this helps you get started!
Andy
