Don't overthink it. Doug's (GDJMSP) advice is golden. The 5x B&L will give you good lens for grading. If you want to look for hairlines and mintmarks, you can get a lighthouse loupe 10x. They run between 10 and 15 USD.
i agree 100% but when looking at some old or hammered coins . it does make a difference. its that small detail that sometimes counts . for those reasons i use a 30x21 loupe.