James I have fitted two Jolley B60 kits. All I wanted was the module although he seems keen to sell you a special coil. I assume that is one with a lower resistance now that the module can switch a higher current current & give more HT output. I assume this is a 12v coil so the ballast resistor should have the same resistance to ensure the coil is supplied with 12v.
I don't see that always striving for a higher voltage is the ultimate goal, because it can encourage leakage through components made 50 years ago such as the rotor arm & HT cable although cable can be modernised.
I just wanted an HT system to run much as nature intended but with a reliable replacement for the contact breakers, so run with the original coil. I think any standard 12v negative coil will do, easier if you try to get one with screw terminals.