I have the 750x750. Love it, and wasn’t experienced with CNC at all. I bought most of the “extras” so I’ll share my thoughts.
Z-Probe: Love it and use it for all of my projects. I’ve skipped it a few times when using the machine to level a cutting board for example or when trying to quickly batch out some engravings. But I think it’s worth it.
Homing switches: I really don’t use these at all. Mainly since I installed them (during the main build of the machine) I have to adjust the location because where they are installed, as they are past the limits of the gantry. I’ve been lazy and haven’t gotten around to this yet. I did notice that with the DeWalt 611 is somewhat large and hits the Z axis plate (A spacer would be helpful here, but a minor adjustment could fix the issue).
Sideboard: I bought one, and find it completely unnecessary. It doesn’t serve much of a purpose unless you are going to mount your Xcontroller to it. I bought it, if I buy another xcarve I’d probably forego it.
I have the 1/8" collet insert (as other people pointed out the DeWalt is 1/4" shank) I bought what I believe was the inventables super pack which was 1/8" shank ($79).
On the subject of bits. I think it’s ok to purchase from inventables unless you have some already. After becoming more familiar with bits, I’ve looked and purchased from other sources. Amazon is alright, but bits can get pretty pricey. I broke most of my initial bits from the super pack while getting used to milling and operating the machine (Some of this was user error on my part, but at the sametime I feel $10-20 bits from Amazon appeared to be a little more robust)
For types of bits, it depends on what your desired outcome would be, I’d have to say get downcut, and a V-bit or two. I haven’t milled pressed particle board, but the downcut bit will eliminate the worry of tear out.
Bit length will depend on your material, just make sure the bit is longer then the material is deep. Keep in mind a long, thin bit, will loose it’s rigidity. This shouldn’t be something to worry about, just a tip I picked up along the way.
You can pass on the material starter kit. I bought it, but if you have plenty of scrap wood laying around this is unnecessary. I got the mix of acrylics, mostly because I didn’t know where to find these locally.
The dust collection wasn’t offered when I bought my machine (roughly 2 years ago). I plan on making one as there are plenty of options out there, maybe I’ll use the acrylics sample pack. I think this is worth while as the machine does kick up a good amount of dust and debris.
A clamp set is a must, unless you are going to screw into the wasteboard or plan on buying machine screws that fit the inserts on the wasteboard (I can’t remember the size off the top of my head maybe M5?)
I think that covers it…