It is common to see a thread about buying a milling machine with someone saying you should save up to half the money available for tooling.
Wrong. Bad advice.
Yes, in 5 years or so you will find you have spent a lot on extra tooling but you must spend everything you have on the initial machine. If after 6 months you find another £200 or so you cannot easily get a longer table or bigger overall machine so the initial purchase has to last you a while. However as more money does come available you can use it for the tooling.
So put it all into that initial purchase and save up for the extras. For the same reason it is better not to be tempted by a machine with DRO when a bigger machine without the frills is available at the same price. Add a DRO later. Don't forget the electronic gizmo will ony have a life of 10 years but a lump of cast iron just keeps going for a century.
Initial tools
On day one you hardly need any tooling, especially if a beginner. You only need one 1/2 inch (12mm) slot drill and a collet to hold it. About £15 total. A slot drill is the name (in the UK) for a two flute or two cutting edge end mill as opposed to the 4 edged ones. The slot drill is slightlty more versatile as it will do everything the end mill will do but the converse is not true. It can be plunged vertically into the work to make a starting hole and then move along to make a slot. An end mill can only nibble around the edge or make a slot wider, not cut straight in at full width.
Collets
The collet is just one, not a set, not a holder for more collets, just one collet plain and simple. It is likely that your mill has a Morse taper spindle either MT2 or MT3 and those collets are quite cheap. They are not the best method of tool holding but will do for starters and if you are lucky your lathe is the same size so some tooling can be used on both. Do not do the drawbar up tight, just a little more than finger tight is all that is required. It is often difficult to release if you were over enthusiastic. Do not put a cold collet into a hot spindle as it will shrink onto it. Do not bash the drawbar with a hammer to release a stuck collet as you risk damage to the bearings. Wind down the spindle and look for the slot in the side through which you can see the top of the collet. Obtain a taper release wedge to insert here and lever the collet free. (after removing the drawbar).
A few small machines offer R8 as an option but that is not necessarily better. The key advantage is they release more easily but are less common second hand and cost more.
When more funds appear you can upgrade to a collet holder possibly a Clarkson / Posilock or more modern ER type.
Clamping
There is no point in getting a dividing head or a rotary table until you have mastered the basics of milling. The next essential is a means of holding the work on the table. A vice is not essential nor is a 'kit' of hold down bolts. Initially buy ony a half dozen T-nuts of the correct size for your mill table as although they are easy to make you sort of need a few to hold the material to make more. Use these with bits of 'all-thread' (American term for cheap threaded rod from a hardware store as opposed to proper engineering bolts) and ordinary nuts and washers. Later it will be a good idea to get proper studs and nuts because the all-thread is weak and will wear and bend quickly. Check the all-thread in the T-nuts and make sure it doesn't protrude from the bottom becasue this will put extra damaging stress on the table by forcing the nut upwards and can bread the slot. If it is coming out bash the thread above the nut so jam it above the danger zone. These bolts with a few bits of drilled thick plate can clamp most items until you have saved up for a vice. With your single milling bit and clamps you can get qite a lot done and learn how the machine works while you save up for other tools. With luck you will have got a standard drill chuck free with the original machine but otherwise it can come along about now. Some people dispense with a regular bench drill to save space when they have a mill that can do it but it does mean extra swapping of tooling.
Flycutter
Along this path you will find you need next a flycutter.
Vice
Although a vice on a rotary base looks like a good idea most people find they hardly ever use the feature and it wastes valuable height on the table while making things less rigid. A vice with 4in jaws will be about right for a typical mill with a MT3 spindle, or a 6in for a Bridgeport mill. Now you are probably ready for a boring head. This is where you notice that lack of spindle height above the table is a problem and you wish you had spent more on the bigger machine.
Rotab
The next purchase will be a rotary table. Get one of diameter equal to the width of the table. One of the problems with small rotary tables is that tehre is no space to get the clamps on the workpiece. You can use the rotary table as a dividing head which is why that item comes last.
Dividing head
The last purchase