Wow. Bold project. It's gonna take more than just the ABS modulator and modified/replaced brake pipes. At the very least, you're going to need to install wheel sensors at each hub and the ABS "brain" in the electrical box on the right-hand side of the firewall. I assume, but can't say for sure, that all the wiring is already in place and the hubs are ABS-compatible. If not, you'll have to do that, too.
If I were you, I'd probably sell the 520i and buy another E34 with ABS. E34s are so cheap and plentiful in the UK that there's no reason not to simply buy a car with the goodies you want.