John
This is a subject on which everyone has an opinion, and is always a good subject for argument when turners gather for a beer or three - - -
The usual recommendation is for a low-speed 8" grinder with either high-quality aluminum oxide wheels, or better yet, CBN wheels. That usual recommendation also calls for a jig to help hold the tools in place for repeatable grinds - the Wolverine system is popular, but there are others that are just as good. Alternatively, either the TORMAK water cooled wheel, or the Robert Sorby belt system are also highly recommended. Those are fine solutions, but they are all very expensive.
So the important question is how much are you willing to spend?
The main difference between 6" and 8" wheels is that the hollow grind is less concave with the larger wheel. I don't think that's a big deal with gouges, but it does matter with skews - and I resolve that problem by sharpening my skews on diamond plates. By the way, regardless of the diameter of the wheel, the problem with hollow grinds is that there is a ridge at the bottom of the bevel than can get in the way when you use the gouge on the inside of a piece - but that problem can easily be solved by simply free-hand grinding away that ridge
The main difference between low speed and high speed wheels is that the high-speed wheel will grind away more steel than the low-speed wheel, so your HSS tools will last longer using a low-speed grinder. That can be an issue when you are first learning to sharpen because your tendency is to grind too much - but if you focus on just refreshing the edge, that difference is not important. There is a school of thought that suggests that when you are first starting out using HSS tools, you should start with less expensive tools while you are learning to sharpen, and then move to higher quality after you have developed your sharpening skills.
You will also see recommendations regarding the color of composite wheel. Frankly, color is irrelevant - color is a purely cosmetic feature that depends on the dye used in the filler, and can vary between manufacturers.
And the difference between an aluminum oxide disc and a CBN wheel is that the more expensive CBN grinds cooler than the stone, and therefore is less likely to affect the hardening of the tool steel. And by the way - heat is more of a problem when sharpening carbon-steel tools; good quality HSS generally won't be affected by grinding heat.
I use a 6" high-speed Ryobi grinder from Home Despot. It came with 32 and 60 grit wheels - I replaced the 60 grit wheel with an aluminum oxide 80 grit wheel. I also have oil stones, and a selection of inexpensive diamond hones that are actually better than a grinding wheel for skew chisels (IMHO). I have a shop-made jig arrangement that allows me to do repeatable grinds on my grinder. And I have a sanding disc that mounts on my lathe that I can use to tune up edges. That's a fairly inexpensive arrangement, but it works for me.