PIC16F84 don't do much and absolutely need an external crystal+MCLR connection.. 16F628 have comparator, MSSP, USART, no real must for external MCLR (this gives 1 i/o for free), can use it's own internal osc (that's 2 more i/o for free) and so on. More with PIC16F88, and a bit more with 18F1320. and there's still the codespace...

There's some temperature curves in the datasheet you can refer to. I still repeat, unless you need a real timing accuracy (e.g. high speed asynch serial comm), internal osc may be just fine. 16F88 one can run up to 8MHz, 18F1320 up to 32MHz!