For any sound system you need AT LEAST two things:
1. head unit a.k.a cassette player or CD player.
2. A pair of Speakers
If you are not an audiophile (not fussy about the exact quality of sound), then you can go and get a decent headunit. There is guy here going by the name "GOINA". He has two head units with him. Its a value solution, perfectly suited to you I think. Hez also in Karachi. As for his repo, iv dealt with him before for PC stuff and so i'l recommend him.
If you're an audiophile then you'll need to dish out at least 6-8k for a good new Pioneer HU.
As for speakers, most people recommend Kenwood 718s for starters. But here i'l say go to any shop and listen to the speakers before you buy them. Besides, many many speakers come in original as well as copies, including the 718s mentioned here.
You can connect the speakers directly to the headunit but that'l limit the volume a little. a separate amplifier will cost. and for that you'll need wiring+fuse etc too. so additional costs if you go the amplifier way.
Unless you post your requirements and budget, there is no way to give you proper suggestions.....