This is a tough question but will try to answer.
At a minimum a twister/stretcher, this will give you two pin in which you can wrap the string material around. With the two post method i call it you can tag end serve your strings and cables see our Youtube videos.https://www.youtube.com/watch?v=vQRLFTlxQHM
Another benefit of a twister/stretcher it all allows for reserving under tension usually 300lbs.
With the invention of the solo cam bows came a really long string anywhere from 80 to 113 inch string. This can cause some serving issues not being able to serve tight enough, this can be helped with a set of string clamps. The string clamps will hold the material and keep the string bundle from adding twist during serving the middle of the bundle.https://www.youtube.com/watch?v=ubVR_ZvuVSo
4 post set up. A 4 post set up will allow serving of the end loops, by having the string spread out 90 degrees to the strut in a rectangle you will be able to serve the end loops. we normally use a .007" serving Brownell, Duragrip or BCY halo. Some even use spectra braided fishing line.
The difference between swinging 4 post or fixed 4 post. The main difference is when measuring a string layout. The swinging 4 post allows to rotate the pins inline with the strut, once your happy with the length swing the 4 post back 90 degrees to strut and start serving. The fixed 4 post is mainly used by production or PRO builders. With our spread sheet it will allow the user to just measure the two closest pins. This is a time savings for the builder. Once you have served the end loops you can move the bundle to the twister/stretcher. https://www.youtube.com/watch?v=baDyjaApRco
String separators- When twisting a string these will spread the strands apart to aid in keeping the strand separated during twisting. This will help the strand lay down next to each other without getting buried inside the bundle making the string look better. Folks use these for tri color strings and pin stripe strings.
4 post tension unit- This will allow user to add tension during end loop serving. by adding tension you will be able to serve end loops a little tighter without twisting the bundle under the serving. Another benefit is this will help even out string tension around 4 pins instead of just 2.
Measuring tools- Time saving devices, not absolutely a necessary item but makes building strings easier. Some of the items will keep the user from having to mark on the string bundle. White strings will show up any mark put on them. Looks are important to the folks trying to make a living building strings. https://www.youtube.com/watch?v=KPx8T1FyXcs