Not much. DHCP service is probably turned off in services. When you get around to it, you can check there. Plus, you can get to it in a number of places in SBS such as Server Manager. Not a big deal. Yes, you can use your router. On the other hand, if using the router, you could just as easily use static if you need to.
Going to either v4 or v5 shouldn't be hard. You should be able to go directly to v5, but that has been an issue for some or you, can't remember, so trying v4 may work. You never have to uninstall SQL. If going directly to v5, you just install, and it will ask you migrate your data and you're done.
