[Darryl Ramm]

You are in the UK, you are going to need to find an HD IO or HDX card for your Mac Pro. If you've been working in the studio business you likely have contacts that can suggest places to you. There is always EBay.

As already commented you need Pro Tools HD, the HD IO 16x16 analog, a HD IO (or HDX) card and a Digilink software license (if using Pro Tools HD 12.6 or later with a Digilink IO). You need to be looking though the Pro Tools Reference Guide, C|24 User Guide the HD IO User Guide. And you'll need a Digilink and DB-25 cables for preamp input, preamp-out/interface line-in and interface output, and presumably DB-25 monitoring output cables since you are doing surround. It seems like you are mostly just hanging out here asking questions, you are quite likely to run into problems if you don't start reading stuff and working out details.

There is nothing magic about bypassing preamps in the C|24, you just connect up whatever preamp you want to use to the relevant HD IO line-level input, you will need a suitable DB25 cable and/or patch bay with DB-25 in and out to make that easy to do. You'll need the actual patch cables and an extra DB-25 cable. Hard to spec stuff you since we have no idea what other hardware you have. And again you likely know local folks who can help you with this stuff.

Since you are in a tight budget and you have not mentioned any interest in HDX I assume that is a non starter and not needed.