I haven't been to Edisto in many years, but looking at the site maps, it looks like the Ravenel site #27 is your best bet. It's a bit of a stretch from Edisto but based on CapCod's comments, it sounds like you'll be in good shape. I have had trouble monitoring that site with the PRO-106 and RS 800MHz antenna from Kiawah island, just a bit north of Edisto. But I think that has more to do with desense from nearby wireless tower site than distance from the Ravenel site.
A transmission site for the Palmetto P25 Charleston simulcast site is located in that same location as Site #27 near Hollywood, SC. There is an additional site near the intersection of Bohicket Rd and River Road northeast of Kiawah. You'll be able to hear radio traffic from downtown Charleston on that system.
It seems unlikely to me that you'd be able to hear sites from the Beaufort direction, but given that much of that distance is over water, maybe so. Remember that all LE talkgroups used in Beaufort County are encrypted.
Good luck - report back what your monitoring experience is.