I am having trouble choosing the correct PIC to buy. Currently, i need a PIC with RF application and also with PWM.
So, therefore, my first question would be "Are all PICs capable of RF applications?". secondly, " how do i consider how much memory do i require when buying a PIC?"
Prices of PICs differ very little. It is, therefore, needed to consider other aspects of getting a chip, such as max clock, rom/ram size etc.
I would suggest you to buy a powerful PIC and to explore all of its possibilites. Once you are familiar with it, you can scale down your project to a simpler PIC.
I suggest 18F452. It is an excellent chip. PICs smaller than 18F452 are its subsets.