It really depends on what functionality you're hoping to obtain from your network by adding wireless capabilities.
If I'm understanding your question correctly, you're just wanting to allow network access for wireless devices on an existing wired LAN. The product you end up purchasing will depend greatly upon your needs.
- Expected range, coverage, and throughput
- Approximate number of wireless devices anticipated
- Expected traffic types and load
- Impact on existing infrastructure
You mentioned that your existing network is 100meg. Depending on the amount of traffic you anticipate passing through your AP, you may want to consider running Gb AT LEAST from the AP to the WAN. Easiest method is making the logical distance between the AP and the WAN as short as possible. Use an open port on the router or the first switch. This way, if you have a lot of wireless traffic, the impact on the wired LAN would be negligible. Just cap the bandwidth on the wireless traffic to prevent bottlenecking at the WAN. Personally, I'd recommend upgrading to Gb anyway for infrastructure scalability.
Everything I've said thus far is assuming the wireless devices will have internet access. You also need to determine whether wireless clients will need access to LAN resources or not.
Two major questions that need to be answered before I can give a recommendation: how large is your existing network, and are you running managed equipment?