The 400w spindle comes with a 1/8" collet already. We do sell other sizes here:
Here is a support article that should help you choose milling bits:
In a lot of cases, several different bits will technically work for the application. Acrylic is a very brittle plastic.
Here is some highlights of that post that might help:
Up-Cut, Down-Cut, and Compression
The cutting style of a milling bit identifies which way the material chips are removed from your project, in addition to which side of the cut surface is smoothed.
Up-cut bits expel the cut material upward and smooths the bottom of the cut pass. It pulls heat away from the bit, and is a good choice for plastics, metals, or other materials that are prone to melting or burning. However, because of the bit's design and upward-pulling motion, it can fight the clamping system you use with your material, especially if your material is thin.
Down-cut bits push the chips downward (towards the material). This cutting style is prone to melting or burning your material, especially if your job requires many passes. The benefit to this style of bit is that, by pressing down against the material, it can help thin materials remain secured to your cutting surface. Additionally, the top edges of your cuts will be smoother than if you use an up-cut bit.
Compression bits have a primarily "neutral" cutting action and smooths both the top and bottom edge of the cut pass. This is achieved by the lower part of the bit utilizing the up-cut style, and the upper part of the bit utilizing the down-cut style. As a result, these bits are recommended primarily for thicker materials which will be cut through entirely. If your project requires shallow cuts, it may not be the best choice because only the bottom (up-cut) portion of the bit will be used, and the top (down-cut) portion of the bit will not be used to finish the smoothing.
Flutes are the cutting edges on a bit. Fluted bits get their names based on the number of flutes present on the bottom of the bit (for instance, our 2-flute bits all have two cutting edges).
Generally speaking, more flutes on an end mill means a smoother edge finish. More flutes also means there is a smaller surface area to eject cut chips away from your stock material. This means you will need to use a slower feed rate, so the bit has time to properly dispose of the cut materials. By using fewer flutes, you can remove material quicker but will have rougher edges on your cuts.
Certain materials require you to remove material faster than others. For instance, a soft plastic like HDPE will melt if the chips are not expelled from the bit quickly enough. You will want to use a single or 2-flute end mill for a material this soft. For harder materials less prone to melting, you can use more flutes: just be careful with your cut depth, so you don't break the bit by trying to carve too deeply into a hard material.
Fishtail Bits, Ballnose Bits, and Speciality Bits
The tip of a milling bit helps determine its purpose. Fishtail bits produce a flat surface at the bottom of your cut. Ballnose bits create a rounded bottom of your cut. These bits are ideal for 3D contouring or stepped layers, as the rounded tip reduces ridged edges in consecutive passes.
We also sell a variety of bits designed for specific applications. V bits, or V-Carve bits, create a pass shaped like a "V" and are commonly use for detailed engraving or signs. We also sell bits designed for fine-details, engraving, or working with PCB.