davey. you will need 10 core if you plan to fit a fuel sender and fuel pump.
this is the set-up i did on my car it will hopefully highlight what you need on the car light/service wise. the engine loom and ignition switch gear are separate looms but have to interconnect to some degree for switched live etc. etc.
Rear wire requirements
1. Tail Lights
2. RH Ind
3. LH Ind
4. Stop Lights
5. Handbrake Light (also needs extending to front brake reservoir for fluid level warning)
6. Fog
7. Reverse
8. Fuel sender
9. Fuel Pump
10. spare or earth if you have fibre glass tub
Front Wire requirements
1&2. Dipped Beam (requires a wire for each bulb due to high current load)
3&4. Main Beam (requires a wire for each bulb due to high current load)
5. Sidelights
6. RH Ind
7. LH Ind
8. Horn
9. Rad Fan
10. Brake Light switch
another couple of little things you will need to watch out for when making your own loom
1. sidelights must work when ignition is off
2. hazards must work when ignition is off
3 full beam can only be used when the dipped beam is on! but not when sidelights are on, or if all lights are off. the exception to this is full beam flash must work in all on off scenarios other than ignition off.
when it comes to relays you only need to use them when the switch gear e.g sierra stalks/savage switches etc cant take the current load of the device. for my car i have used relays for dipped beam full beam/ hazards for the flasher and fuel pump(switched on and off by engine ecu in my case).
hope all the above helps. making your own loom for the first timer can seem very daunting but its very rewarding when it all works. The plus side is if you ever have a problem you know the system inside out so it should be easy to fix.