Download Pic30_tools from Microchip. They have a free student version. This is a port of the Gcc compiler. Which is a fully featured C compiler.
Pic24 series are fast 16 bit micros, pretty powerfull.
Plain C will do the trick. Look at the include file for the micro for the register names.
The Gcc compiler is the open source compiler from the gnu comunity. It is written and maintained by thousands of people world wide. It has been ported to just about every platform you can think of.
As for conformance and performance, there is not much to touch it. A c compiler compiles c for your platform, but not all c compilers fully comply with the standard. Gcc does.