Most primes are very sharp
Apart from a sigma I read about
Well, its very sharp. Infact, the sharpest lens EVAR in the centre, but towards the border its crap, and its got loads of CA and PF ahahah

Yes, I would say straight away to buy the 50mm F/1.8, and if your buying pentax, the 1.4. Its one of the best lenses around, and £160, well built too.