The 785d will not trunk-track APCO-25 systems with 9600bps control channels, which is the kind of system used in Charleston. So you want to avoid that. You may find the 796d performs poorly on the Charleston Co. TRS because it is a simulcast digital system that uses CQPSK modulation. Even the newest radios struggle with it, but I would definitely test out the 796d before you buy. Some have said the HP-1 is among the best at decoding simulcast systems, and it is easy to get up and running for folks new to the hobby (easier programming) but may be limiting for advanced users.
I agree with Larry in the value of finding someone else in the area to close your radio from.
The BCT8 is not digital, so you'll be limited to SCHP, MUSC, DNR, etc. You'll miss all FD, EMS, SD, and PD traffic from Charleston Co, Charleston City, MP, NC, etc.