Sure thing. Pm me some time with your email address and I'll send you links. Save yourself $100 some and skip the DeWalt 660 and go straight for the 55mm spindle. I'm using this one.
I'll dig up the thingiverse link to the spindle mount that I'm using and send you that too.
Water-cooled spindles are quieter, but they require more parts to make them work. In general the cutting noise of the bit against the material is louder than a VFD spindle anyway, so the noise difference between a water-cooled and air-cooled VFD spindle is kinda a moot point. Both are very quiet. However I will say that my air-cooled spindle heat-soaks really badly on longer jobs. So if you intend to run hour+ jobs it might be worth going with water. On a machine that small though, I would think most jobs would be short enough that air would be okay.
The spindle+VFD route is almost certainly going to exceed your $200 budget. At first it seems kinda possible because just the VFD and spindle can be bought for not much over $200. But there is a lot more stuff too. Shielded VFD cable to go between the VFD and spindle is pretty expensive, yes you can just use regular cable but it will shed a lot of noise which can cause other problems. And you might need some various relays or PWM boards to interface the spindle with your controller. You need a clamp to hold the spindle. Etc etc, it all adds up. Its a good route but if you are strictly on a $200 budget, you will prob end up spending more than that. I would recommend getting a slightly oversized VFD, ie if you get 1.5kw spindle, get a 2.2kw VFD, it makes the VFD work less hard so it can last longer.
To actually hit your budget, you could use a handheld trim router like Makita RT0701C. It is 65mm diameter and you can get aftermarket collets for it, so you can run 1/8 or 1/4 bits.
Another option is to get a Carbide Compact Router, it is basically a Makita knockoff (so it is also 65mm diameter) but already comes with the collets included. They also make a version with an ER11 collet, so you can run all different bit sizes rather than just being limited to 1/8 and 1/4. But it is more expensive.
Be warned, trim routers are loud. But again, just cutting anything is going to be loud, and if you run a shop vac for dust collection that is loud too. So regardless what spindle you use, it is prob gonna be pretty loud when cutting. But yeah, trim routers do scream.
There are also the smaller DC spindles like this that are commonly seen on 3018-style machines, but most of these I have heard mixed reviews on. I would probably just use a 65mm trim router such as the Makita or Carbide. Using a router also gives you a clear upgrade path as you can easily drop in a 65mm VFD spindle in the future if you want without needing to change your clamp or Z axis.