Price difference between PIC24 and dsPIC33 is very small. dsPIC33 is much more powerful. You may consider using the dsPIC33 instead of PIC24. The microcontroller core is more or less the same in both. Instructions are also mostly the same, besides the DSP section.
For the PIC24 series, one of the most popular is PIC24FJ128GA010. Many of the trainer boards for PIC24 (most that I've seen) are built using the PIC24FJ128GA010. Bear in mind that it is not available in DIP package. It is available for simulation in Proteus. Among the small PIC24s, you can take a look at the PIC24FJ16GA002. Download the two datasheets and select the PIC you think is fitting for you.
For the dsPIC33 series, there are 3 common series: GP, MC, GS. GP is for general purpose, MC is for motor control, GS is for SMPS.
There are other families. I talked about the PIC24FJ which is available for simulation in Proteus. There are the faster and enhanced PIC24HJ and PIC24E series along with the new dsPIC33E series, but simulation is not there for Proteus.
Hope this helps.
Tahmid.