Ok - first up gonna need some more info about your car.
There are two stereo types - the 'round pin R17' and 'flat pin R40'. You need to know which you have in order to buy a compatible one.
I think you can get a harness to put an R17 in an R40 car, but not the other way round. Or maybe it's the opposite. I forget.
Anyway, if you want to put an aftermarket stereo in, then just go for it - it will look proper toss but will probably sound better than the factory unit - UNLESS you put in a nice amp on the back of your stock head unit. I put in a pair of rockford fosgate amps on the back of a david navone amp - and it sounds boss. I added speakers to the rear doors, changed all the other speakers etc and it's wicked.
Having said that, now my electrics are fucked after hitting the electrical system too hard with bass.
Maybe it's not the best idea to listen to me!!