I had problems with DHCP on my old WiFi router too, which I "solved" by putting a static IP address in the DS configuration.
Regarding the two packages, they are exactly the same apart from the form factor of the expansion RAM cartridge. The "normal" DS RAM cartridge works with both models of DS (but sticks out on the Lite), and the Lite edition only works in DSLite, but doesn't stick out, which is much more elegant.
Regarding speed, it's slow. It's definitely usable for most stuff, though (Google, GMail, Yahoo, blogs, forums...). Browsing through more intensive pages (Ebay, IGN...) starts showing the limits of that browser. To be fair, IGN is also slow as molasses on my Core2 Duo + 2GB DDR800 RAM, so I'd suspect there is a problem with the site itself.
Usability is okay. I dislike the fact that it doesn't keep cookies (it does have bookmarks), and several formats are not supported, including Flash. So it supports less stuff than the PSP browser, but text entry is much easier (virtual keyboard + writing recognition).
Readability is pretty good considering the low resolution of the screens. The dual-screen mode is very nice, and you can change zoom level on the fly.