Your head unit doesnt do 50 watts. More like 10-15 actual watts. Never beleive 50 watt claims on any head unit. Only head unit that actually does more than 40 is the sony MEX 100bt which specifically has a real amplifier stage built inside rather than the cheap internal amp chips that every head unit on the market uses.
What I would do is focus ONLY on the front . Anything in the rear is wasted money and screws up sound stage. However if you dont give a fk about any of that and just want something decent that works for now, then just get a coaxial drop in replacement like that rockford 6.5 and 6x9 deal that works well on head unit power and can take power later on.
When you are ready for a real front stage later, you can do it right then and just keep the rear 6x9s on head unit power and focus all the amplifier power on the front components later.
These are it.
6.5 + 6x9 speaker package