On projectors - biggest thing as you already know, is that they be legal. Lightspace from Walt and Able are both good choices. I don't know that Swisslas or ECS are. You should also consider speaking with Adam Raugh who replied to your thread above about X-Laser's offerings and, specifically the Skywriter HPX Tours, both 2 and 5 watt. You mentioned not knowing about going ILDA versus DMX since you'd like to use Artnet. They are two really separate animals. If you were just thinking beams, then it could potentially be either one with the advent of the Mercury system that X-Laser has. However, if you're thinking about the 3D mapping, then you really need to think ILDA and Beyond. Now.... Beyond itself can be triggered via DMX/Artnet from what I understand but, I don't know exactly how it works. There are those on here that do. I'm sure with the pyro that you're learning about, that's why you're thinking alog the lines of DMX/Artnet but, ILDA is ultimately going to give you the most control and leave you open to more advanced things such as the 3D mapping.
DMX/Artnet is really more for lights, pyro and that sort of stuff and ILDA is traditionally more for lasers however, the Mercury system is changing that and making lasers more accessible to lighting designers who are more in the DMX/Artnet world. There are still limitations with it but, it's significantly more advanced than it was even a few months ago, thanks to Mercury.
Invest in Beyond and a single FB3 just to get started. Update Beyond at Christmas time when it's on sale. You can make a decision about FB4s versus FB3's later down the line. The FB3 will still be useful if you lean toward FB4. The APC40 is really just an extension of what you can do with a mouse for those people who like to have the tactile feel of buttons and sliders versus having to point and click. The other advantage is once you have developed muscle memory with your hands and where buttons and sliders are, it allows you to keep your eyes focused on the lasers and not a computer screen and where to position a mouse. So again, you don't necessarily need an APC40 right out of the chute in order to start learning. It's something performance based that you can grow with. I'm personally not that good with that skill. I still need to look at the keys while typing or, playing piano or something tactile like even the Z-5 analog console so, I mainly stick with a mouse and sometimes make use of my touchscreen. My APC is a dusty thing I wish I had the time to get my hands used to.
Haze/fog etc. Don't go cheap. Also... think about it the way you might think about a printer. What is the cost of the consumables? i.e. the printer ink/haze or fog fluid? I don't give a damn about what the printer costs.... the real question is... what am I going to spend on ink every few weeks? First decide, do you want to be buying fluid for $30 a gallon every few weeks or... a gallon every few years? If the answer is every few years, then maybe the machine that's initially $300 more isn't such a bad option anymore.
Oil based versus water based.... an oil based machine is going to give you better hang time meaning you'll run it less and while people try to say they're "messy", I run my DF-50's all the time and have never noticed any build up of oil on anything. I've also not had to add any fluid to the machines for literally a couple years. They only need to run for short periods before and during a 3 hour show. $25 per gallon. A water based Smoke Factory tour hazer on the other hand, runs constantly and, once you turn it off, atmosphere is generally gone within 10 minutes. Yes, it's economical in it's fluid use but, it's still $105 per gallon delivered to my door. Drawback of the DF-50 is it's noisier in its operation and, some venues specifically say, no oil based hazers for perceived fear of oily residue. You also can't control the volume of output - it's either on or off. The Smoke Factory I can turn down to a barely detectable level and fill a room slowly with haze with no one even noticing. So, all sorts of things to consider.
(Just a few answered questions/opinions for you.)