800 feet on wireless DSC? I wouldn't trust it.
800 feet with a buried cable and a power supply on the other end? Go nuts. I've done it before. As I recall the maximum cable length for a DSC combus is 1000 feet with 22 gauge wire.
Double checked the manual for the 1864, and the maximum Keybus/Combus length is 3000 feet, that's total length of all keybus/Combus devices. If you're just in a house, I doubt even with the 800 feet to the barn you'd be near that 3000 feet limit.
With the distance, voltage drop would be an issue, so my personal recomendation would be to have a power supply at the barn end, so none of the barn devices are drawing power from the house. The PC5200 Powersupply/Relay Output Module gives you 1 amp of power, and the benefit of having the power supply easily monitored for AC power loss and battery trouble.
With four expansions already, you can only add three more (giving you 21 zones). So not much room for further expansion. You also can only have a total of 8 keypads on the system. So as long as they don't already have more than four keypads in the house, you can add four in the barn no problem (again, using the power supply in the barn for their power, not the house).
If you do got the second system route, DSC is a "modern" system. Maybe not quite to the level of ICT (in the sense it doesn't do Access Control) but it has it's own IP modules and GSM modules for monitoring as well.