skip the sony HUs, many users wont recommend u taking those ...but I heard those new ones are better made compared to last time but not sure laa...

if u want slot ones like u said sony before, better go for those entry level pioneer (seeing from ur budget RM300-500) HUs like 2850, 2950, 3850, 4850 or maybe better still 5750...

Im not sure whether ur preference includes motorised front panel where the slot is hidden like 4850 and 5750 but if u want direct loading disk slot, then 2850, 2950 and 3850 shud be good for ur usage...

are u using myvi also ? if yes, then firstly if u want to use after market HUs, then u have to take out the OEM standard stock CD player with the bracket or frame it comes with, change to another frame or bracket which will enable u to put after market HUs .....for those brackets, I have seen from RM60 till RM150 also got, depending at the quality and where its sold....

for a normal HU which can play MP3/WMA discs along with the USB port capability for thumbdrive, can get almost any brands there is except those chaplang ones including brothers ...I havent seen pioneers one just yet, but I think JVC and kenwood already have those...between the 2, go for JVC...