Budget of RM800 is a little tough to work with for HU + speakers.

It would just be better to get a HU based on that budget, or, if not, just spend that budget on the HU and a pair of better front speakers(preferably of the 6.5" size, no problems installing it for a skilled installer), forget about the rear speakers, leave it as stock, or disconnect it.

That's a pretty decent beginner HU. Pricing should be around RM500-600.

Save up some money for speakers, if you plan to get component sets, you'll need to budget around RM700 and above for a decent set inclusive of installation and all.

You might also want to add on some sound dampening material when you install your front speakers, at the least minimum, deadening the 1 square feet of panel where the speakers are installed on since you'll be removing the door panels anyway