You can just buy a car-mountable LCD (which is just mounted on the dashboard of your car). A Liliput brand would be relative good yet cheap (just dont leave it under direct sunlight). Cost of that would be just 4000-4500 brand new in Carriedo. For the rear cam, I believe there are plate mountable types that can be bought in Banawe or Evangelista. It has a mounting bracket wherein you will just mount it together with your license plate. Cost should not exceed 1000. Overall it's just about 5500 max + 500 installation perhaps (500 is exag but if you would like the power of the LCD to be wired somewhere else other than from the cigarette lighter, or you would like the wires to be minimally visible).

You can also go on changing your headunit with an in-dash LCD or 2 DIN LCD unit. Preferrably ung 2 DIN kasi ung LCD pag open ng HU nandun na agad, samantalang ung in-dash kelangan pindutin mo pa ung pull-out button para lumabas ung LCD. The generic china types would cost about 7000-10000 for the in-dash 1 DIN LCD and 12000-25000 for the 2 DIN. May nagooffer din sa Tsikot Virtual Banawe ng ganito na mag-add ka na lang din ng 1K sa price ng headunit at may rear camera ka na. This is more expensive but the good part is that less wires, less hassle of removing your LCD whenever you park in the open in broad daylight, and you get a cooler HU (unless your current HU is already awesome).

As per installation, you can try checking the "saang shop sa Evangelista" (correct me if I'm wrong) thread. Hope this helps.

btw, DIN refers to the size the HU will occupy in your dashboard, and HU means the head unit or the device you use to tune in to the radio, listen to your cd's, etc.