I currently have a TI-84 Silver Plus C Edition. Is an upgrade unnecessary or would something like a TI Nspire CX be better?
TI-84 Silver Plus C Edition should be fine for you. You would do most programming on Matlab not on a calculator.
Eh, you don’t really need a fancy calculator. A cheap TI-30X IIs will probably suffice for most, if not all, your needs. Many professors won’t allow graphing calculators on exams, and free tools/software (like Wolfram Alpha) are available to handle everything your graphing calculator can do and then some.