Sharpening is one of those subjects where if you ask three people you'll get four opinions.
That's profoundly true!
The important point is that if it works for you, it's perfectly ok. So the question is really what works for you? Getting an answer could be trial and error. Capt Eddy jokes that you can use anything to sharpen - even an old brick.
The difference between high speed and low speed is theoretically the speed of rotation, but a more important factor is how quickly the grinder removes metal. You can sharpen just fine on a high-speed grinder, but you must learn to use a light touch.
The difference between 6" and 8" is theoretically the diameter of the wheel, but it also is a factor in how quickly metal is removed. Also, a 6" grinder will produce a more noticeably hollow grind than an 8" wheel. That may nor may not matter to you.
The wheels that come with grinders are so-so - typically, they are 32g and 60g. You can easily replace a wheel, and the important factor in choosing the replacement is the grit. Color is totally irrelevant. Aluminum oxide is the preferred choice, and 80g-100g is better for sharpening. CBN wheels are really nice, but also really, really expensive. But they aren't really necessary.
As others have said, having a jig is helpful in controlling how you sharpen your tools, and that's more important if your grinder tends to remove metal quickly. You can buy a jig, or you can make your own.
You can also use sandpaper to sharpen tools. In fact, one tool manufacturer (Sorby) uses fancy belt sanders (called lineshers) for in-factory grinding.
Some tools are best sharpened using either conventional oil/wet stones, or diamond plates. For example, I only use diamond plates on my skews.
Having a grinder is helpful - that's the conventional tool for sharpening, and having a grinder is also important if you get into making your own tools (including carbide tools where you have to shape the bar that holds the carbide cutter). I'm glad I bought one and I use it to sharpen my gouges.
Diamond plates are also handy, and if you go with the brand-x variety, not that expensive. Harbor Freight has a nice four-sided block that is useful. I use it on my skews and bench chisels.
Diamond paddles are also handy and inexpensive (in the brand-x variety). Useful for sharpening drill bits, especially forstner bits.
And diamond cards are also very convenient to tune up a skew or a forstner bit.
I also have a 5" hook and loop sanding mandrel that I can attach to my headstock to make a disc sander with interchangeable grits. This is good for reshaping skews if you inadvertently clip a corner on a chuck (DAMHIKT), and also for sharpening gouges (you have to learn the technique of constantly rotating the gouge while sharpening on sandpaper)
