you can get them from harbor freight for less than 20 bucks. i have about 5 with different grit wheels on them because saves times changing disks. mine are straight but you're probably better off getting a angle one. Also have high cost Dewalts and Makita for the grinders I use all the time. Always use eye protection.Chemically eliminating the rust (via liquid or hell product) could reduce risk of either leaving behind rust particles or contaminated metal.
I also don't own a flap disc or angle grinder tool.....yet
Are you using a 90 degree angle die grinder or a straight grinder?
I guess using chemicals would eliminate rust from reoccuring but i havent had that problem .......yet.