Good for you....Love to see others get involved in the Metal Working trade, Hobby or Professional.
The Grinder is the most used single piece of equipment in the shop!! No joke, you use them for everything. I personally like to use the 4-1/2" side grinders for all my sanding and grinding needs. The bigger 7-8" grinders are just to big and heavy for most of what I do.
I have a different take on grinders, I like Craftsmen, Dewalt , Makita and bosch. Personally I try to avoid the ones with the safety latch on the side by the triger. The ideal is a good one, but if you use the grinder a lot, it gets old having to push in the safety, before the trigger. There are many very good grinders out these days, so avoid the Harbor freight junk, it's horrible to use and last about one good sized project.
I have to get a lot done in a short amount of time, so I have 8 - 4-1/2" side grinders, 5-Craftsmen, 1-Dewalt, 1-Maketia, 1-Bosch. I just keep them all loaded with the tool I use often. Like one would have a backing disc and 36-Grit Sanding disc, one a 80-Grit and one a 120-Grit. One grinder would have a grinding disc and another one with a thinner disc and one with wire wheel. The point is, it takes much less time to just grab another grinder, then it would to constantly remove and re-load the proper sanding or grinding disc. I understand you may or may not be able to adopt this system, but it works very well.
I also have a very nice small 4" grinder that I use to get into tight areas with, this grinder is very much smaller then the 4-1/2" ones, and I like it very much, it's an older Maketia.
Sanding Fiber Backed Disc, are the number one item I use, I carry a large selection from Grit 24 thru 120. For final sanding, like before painting then you switch to a DA sander and go with the 80 thru 600 grit paper disc.
If you look around on the Web, you will find site that will save you aleast 50% on your Disc, you will have to purchase them in 25, 50 or 100 quantities, but they get used up quickly, and should be bought in larger quantities anyway. Good luck.