Most head units rarely put out 50 watts! they normally mannage about 16 watts.
Speakers need to be driven and if they say 50 watts output then get an amp that can push out a little over 50 watts so your speakers can be driven properly.
Its like a hifi system and the speakers say 100 watts and you buy an amp that is 50 watts ? you wont be giveing the speakers the correct power and you will be under driveing them.
As for the disco speakers go to your local car audio shop and show them the speakers, they should be able to find some decent quality ones that will fit your disco.
Stock speakers are normally rubbish and need to be replaced by quality ones, and then you will hear sounds you have never noticed before.
Most people just have a pair of quality speakers in the front and a sub in the back for bass, the speakers in the front will be loud inuf for the front and rear passengers.
Im doing the same but ive got two pairs of 6-9s and an amp to drive the speakers properly, later i will get a sub and just have a front pair of quality speakers.
Hope this helps
Darren