which router for WDS?

If you want to set up a WDS, my suggestion is that you use an Airport Extreme Base Station as your wireless network's main base station. You will save yourself a lot of potential setup problems, since Apple's Airport Setup Assistant greatly simplifies the configuration process for you. The Airport Extreme Base Station has only a single built in ethernet LAN port, but you can easily link that single port to as many additional ports as you require with the use of a low-cost ethernet switch.

There are only three or four specific non-Apple wireless router models on the market that support Apple's implementation of a WDS - WDS is implemented differently by each manufacturer (and is not a standard WiFi protocol). The most popular of those few available models is the Linksys WRT54G, and detailed instructions for using it in an Apple WDS are available both here and elsewhere on the internet. As for D-Link - none of their wireless router products support Apple's WDS implementation.

You may find that a new model wireless router will solve your existing wireless range problems all by itself, without having to resort to setting up a WDS with your Airport Express. If that happens to be the case, then it really doesn't matter what wireless router model you buy - since the Airport Express could be used in client mode with any make/model of wireless router.