I researched computer mice a few years ago. I think the Logitech G400s is the best mouse on the market and costs ~$50. It's both the most comfortable mouse I've ever used and good by every measure of features and performance.
However, I also used to have my hands and forearms hurt from spending all day on a computer or at a desk. The three biggest things to fix this are:
1) Lower your hands as much as possible. My keyboard tray at work brushes the top of my thighs. Make sure you have a way to keep your screen(s) at a proper height when you do this. So, if you're using a laptop, get an external keyboard and mouse plus a laptop stand.
2) Make your keyboard flat, not tilted up in the back.
3) Most importantly, lift weights. For this specific problem, the best exercises would be heavy deadlifts, neutral grip pullups, chinups, overhead press, and Pendlay rows, in that order of importance. Don't waste your time with wrist curls or hand grippers or bicep curls or anything with dumbbells. Trust me, I've tried all that stuff and found it to be vastly inferior. Lift heavy with a barbell. If you can do more than 5 reps in a set, it's too light, especially for deadlifts. If you get stronger, even shitty ergonomics aren't that big of a deal. To learn how to do this stuff, the StrongLifts website, the book Starting Strength, and various YouTube channels are great to get going as a beginner.
