You can certainly disable DHCP on SBS 2008. I'd probably just deactivate the DHCP scope created by all the "wizard" garbage. That ought to keep the "health check" functionality and such happy, but won't actually allow the server to hand out IPs.
Just be sure that whatever does hand out IP addresses hands out the proper DNS server addresses (ideally the SBS server if this is a small single-server environment).
It's probably slightly inefficient to have the DHCP Server service sitting there running w/o actually doing any work, but I'd rather do that than chance that some of the idiotic "helpful" functionality in SBS gets in a huff that it sees the DHCP Server service not running.
(You can tell that I don't think too much of all the "handholding" in SBS... >smile< For me, the SBS products are just a way to get an inexpensive Exchange license bundled w/ Windows. I admin them like "normal" servers and try to ignore the extra junk that comes with them...)
You probably ought to set the group policy setting to allow client computers to register their own PTR records into DNS. By default, the DHCP Server service does this for client computers (and I have no idea why Microsoft designed it that way, so don't ask... >shrug<). The setting is under "Computer Settings" / "Administrative Templates" / "Network" / "DNS Client" / "Register PTR Records", and you want to set the "Register PTR Records" drop-down to either "Register" or "Register only if A record registration succeeds".