If it takes more than 10-20 minutes to sand a fairly large room between coats, 60 after the final, with a sanding block, you are applying too much mud, with too little patience. I have a 24" Marshal trowel which I am use to, it will make just about any flaw disappear, best part it leaves little excess compound. Agree with above poster, hand sanding is best.
If you plan on anything but flat paint, during your final sand you need both incandescent and fluorescent lights to inspect, if you can see or feel a flaw with either, you will see it in the finished paint coat in any light. Your eyes catch about 85% of the flaws, your finger/palm the remainder.
Save your $500 for more useful tools, unless you like buckets of compound dust. If you get mechanical tools, I doubt you will lean to tape properly, so it will cost you $500 for mechanical tools and hundreds in lost labor in extra sanding.