Must Have:

Approved Rollcage
Underbody protection for fuel tank (Get a sumpguard if you want any chance of finishing an event)
In date Seats (7 Year lifespan, add 2 to the expiry date on the seat for UK events)
In date belts (5 year lifespan, 10 if a six point belt on a specific post 2016 list)
In date 4L plumbed in fire extinguisher (10 year lifespan, must be serviced every 2 years)
In date 2L fire extinguisher inside the car with anti submarine clips (as above, 10 year lifespan, must be serviced every 2 years)
Electricity cut off and Plumbed in Fire extinguisher pulls, one set below the windscreen on the plastic cowl, one set on the interior within reach of the driver and co driver with belts on. Must be labeled with the correct safety stickers. Interior electricity cut off should be a switch with on/off marked.
Mudflaps for each wheel, dimensions will be in the blue book
Towing Eyes clearly marked with a contrasting colour at both ends
White square on the side of your car to put your number on, dimensions listed in the blue book
Windscreens must be laminated glass, if rear side windows are made of glass they must have safety film on the inside (must not be thiccer than 100 microns)
Window slides on both driver and codriver doors
Metallic brake discs only
Brake bias adjuster must not be within reach of driver or co driver
OK board (Board that says OK on one side in Black and SOS on the otherside in Red, should be A4 size minimum, buy one online).
Warning triangle
First Aid Kit and two high vis jackets (not necessary but highly recommended, will avoid scruts having a moan at you).
Spill Kit
Approved Helmet with FHR posts
Approved FHR device (Hans or Simpsons)
Approved race suit
Groundsheet for servicing, there is a minimum size listed in the blue book (bring some axle stands too so when your jack fails your head isn't crushed like a dweeby watermelon).
No airbags
Full list of minimum weights are in the Stage Rally section of the blue book (1983-now for 1.4 is 700kg, 1.6 is 770kg, 2L is 850kg)
Permitted tires from section L of the MSA blue book
If you have a sunroof it must be sealed
Belt cutters within reach of both driver and co driver
High intensity horn
'All new build cars must be equipped with the facility to enable a fuel sample to be taken. For fuel injected cars the facility must be a dry break fuel sampling coupling,approved by the FIA, Competitors must carry and make available a 300mm minimum length of hose to which, where necessary, the appropriate mating part is to be
attached.
Max sound is normally 100db at 4.5k revs
Obviously must be MOT'd, if you do anything outside of single venues make sure you're taxed and insured too.
Get your car looked at by a scrutineer before you go to an event to get your vehicle passport. You must have a vehicle passport to do events, info on how to do this is on the MUK website.
When going to an event you'll need your vehicle registration document, MOT certificate, MUK (formerly MSA) vehicle passport, Stage Rally License, Club membership card. For multi venues bring proof of vehicle tax and insurance too.

Highly Recommended, most recommended at the top
Intercom system (Stilo>Peltor>Terratrip, though stilo costs a lot more, terratrip is the budget choice)
Something to hold spare wheels in with a jack in the boot
Strengthened chassis
Some sort of additional lighting should you do night stages
Competition brakes
Replacement competition steering wheel (make sure you wire your horn up to another button if you do this)
Competition suspension
Hydraulic handbrake
Competition Wheels (must be within 2 inches diameter of the wheels that came with the car) and Tires so your rims don't break and you don't get as many punctures (muh kevlar sidewalls)

DONT FUCK WITH THE FUEL TANK ITLL MAKE GETTING YOUR CAR LOG BOOKED TEN TIMES EASIER, leave the battery in the engine bay too. Spacers are gay and cause problems don't use them, I'm speaking from experience/

Remember when looking at the blue book regs to make sure youre reading Stage rally regs and not Road Rally Regs (Stage Rallying is from Section R.21 down). Make sure to read sections J and K as well.

This is a basic guide and is in no way comprehensive, read the blue book and event regs to make sure your car is fine for competition use.