Getting an external preamp depends on how much quality you want to have from your recordings. If you're recording spoken podcasts or speeches I think that you'd rather want a low ground noise level, which maybe will be tough to reach with the recorder's own preamps, so maybe getting an external preamp would help. Nevertheless, that would bring your budget significantly up...
Regarding the mic, Shure's SM-58 is widely regarded as one of the most reliable and best quality/price mics out there for recording voice. Keep your eyes peeled and be careful for there's a lot of Chinese fakes out there