Just get the 0-150mm vernier caliper to start with. A reloader uses a vernier to check OAL and seating depth. I would also recommend a stick on tape measure.
A 0-150mm M-Sure vernier is £23+VAT on the link here for their site:
M-SURE MS-220-150 Standard Digital Caliper MS-220 Series 0-150mm (0-6")
In my reloading the vernier is used every time, in fact several times on each cartridge, (the case length, neck diameter, bullet length, overall length).
The tape measure, get one that sticks to your bench: it is great for sorting things fast. Sorting bullet length, bullet weight, COAL, just take a measurement with your vernier or scales and put the bullet or case next to the tape measure for that. For example, if bullets should be 26mm long, then I will get a range from 25 to 27, so put them beside the fraction on the measure divided by 2 (so 1 metre of measure covers 25.00 to 27.00). I take a pack of 100 bullets, sort them by length or weight or both, and remove outliers. Doing that gets rid of my fliers. You would be surprised how far out some bullets are from the average.
A caveat: I reloaded a lot years ago with a Texan chap in California before California took up fentanyl and uber-liberalism, then got back into reloading fairly recently after returning to the UK. All of us are different. I am an engineer so may do things one way or in more detail when others just want plinking rounds or the odd deer round made up, where even an old Lee hammer driven reloading set is good enough. My reason for getting back into reloading was purely precision shooting as an engineering activity: some engineers retire and make watches, violins or fancy optics, others like tuning metal to shoot.
Beyond a vernier caliper and tape, further measurement tools are needed only if you are REALLY into precision shooting or are modifying things, which some on here are, so for them this is exposing how I do it:
- When measuring cartridge concentricity, wall thickness or barrel thickness, I use a digital dial indicator (plus a mandrel or pin gauge set for case measurements, or for barrels made a brass rod with a ball pressed into it to go inside the barrel). Dial indicators are fixed to jigs that clamp to flat plates on granite blocks, or clamp to the lathe bed, so the indicator is the least significant of the costs here and all these are tools that I had anyway.
- I fell into the neck tension spaghetti due to annealing, and FLS die that were giving me wild variations in tension, right down to zero. Forster, Wilson, 21st Century make good die. Somebody else's FLS die drove the need for tools and time. Far cheaper to buy decent die or a mandrel. In measuring neck tension, I use pin gauges and a 0 to 1 inch micrometer, or a small bore gauge and dial indicator depending on what is closest to hand. For private use, Chinese pin gauges are fine as they don't move and last forever. Once one has fallen too far down the neck tension rabbit hole and need a holiday, then you have a load of spend on mandrels etc, a press with a pressure gauge or as I did, fit a load cell to your press, connected to an Arduino connected to a laptop and python script to read it etc to plot strain (distance) over stress (pressure). The M-Sure linear scales have a digital output that the Arduino or PC can read. AMP do one off the shelf for a price, or just ensure the neck is 3''' under the bullet size by using a mandrel and you are good to go.