I have two rally Minis, a 1964 Cooper 'S' which is a full-on historic G'p 2 FIA spec and a 1990 Cooper 1275 which is a road rally car originally built for the up-to-1400 cc Endurance Rally class back in 2003.
The 'S' runs twin H4's and I have 3 throttle springs, whilst the 1990 car has a single HIF44 (at the moment, I'm going for twin HS4's when I have the time to do it} with two throttle springs.
Having suffered from the failure of a single spring on a well-modified 850 (it was a very long time ago!) whilst on a competitive section, I always want a 'fail-safe' set-up.
On the subject of sump guards and oil coolers, on the first 998 Cooper which I bought new in 1964, I fitted a really good full-length sump guard and then I drove it up the M1 at a constant 80+ mph (no 70-limit in those days) and after about 15 miles at just over 5000 rpm in top, the oil pressure dropped to about 35 psi. I fitted a 10-row cooler and the oil pressure was then fine.
Don't forget to fit a flexible map light and a 12 volt power socket for a map magnifier.
If you get more serious you might want to do some more reliability mods. Typically competitors run the brake, fuel and battery lines inside the car, fit twin electric petrol pumps wired in parallel using separate circuits/switches and do a bit of weight reduction.
However, the main thing in rallying is reliability. make sure everything is in good order. You can do little tweaks like having a spare coil already mounted, twin distributor condensers if you have to run points ignition, as you do for historics. I don't have the condenser mounted inside the dizzy cap, I have a pair of them mounted on the RH front bulkhead triangular reinforcing next to the brake and clutch master cylinders. Waterproof the distributor, plug leads and dizzy cap with a liberal coating of Vaseline or non-conductive grease.
The tests are a bit hard on the transmission and the engine will rock about when going into reverse and then forward again. Have good engine mounts and ideally fit an 'Ultimate Engine Steady'.
Just do what you need as you get more into it.
If you wish I can send you a build sheet for my 1990 car and the 1964 car, but I'll need you to PM me with your email address.