Hi Jay
Basic tools are:
Oil pressure gauge
Henry hoover with Hepaflo bags, wire brushes and soft brushes
FGA
Smoke pump
Good set (Wiha are good) of Hex keys and spanners
CD11 forms (The HWOS forms are good)
To start with you can get some general nozzles if you wish. The best way is to get as much information about the boiler from the Customer before you go, such as make, model and problem. This way you can look at what part is required etc before you go. How much stock you keep will depend on how good your local merchants are. If they stock most things such as burner oil pumps, photocells and nozzles, you should be able to get by. Buy common photo cells like the Riello RDB long photo cell and the MZ770. You don't need a massive selection of transformers, motors and control boxes to start with. This is something you can experiment with and find what works to your budget and the distance you have to travel to a Customer.
I have known heating engineers get by with very little in the way of stock. I have a good range of Oil pumps, over 200 nozzles, a range of capacitors, transformers, control boxes, electrodes, burner motors and other bits and bobs such as filters and flexi's. I didn't start out on day one with all this and it has developed over the years. Your Customer base will dictate your stock as different areas have different boilers. If you are just starting out in oil build up gradually otherwise you may find you buy things you will never need.
What you will need if the ability to learn quickly. If you do not have much repair experience you will have a steep learning curve or Customers will not be happy. This is where having some basic stock comes in handy as you can easily check if a part such as a photo cell is faulty by putting a new one in for a few minutes to try.
I think as you have a FGA already, you should be able to get enough basic stock for £500 to get started. If money is not an issue buy the HWOS select kits.
Get the Riello pump kit
Danfoss Pump kit
photocell kit
A selection of capacitors and electrodes
A couple of common transformers such as EBI and Riello and maybe a couple of control boxes
All the other bits you can get as time goes on
Good luck!