So I recently purchased a new car, and I've got my ~3 year old Radio (http://www.sonicelectronix.com/item_22924_Kenwood-KIV-BT900.html) and my year and half old amp (http://www.sonicelectronix.com/item_23877_MB-Quart-ONX4.60.html).
Would It be a good idea to just reuse these guys or find something new.
My restriction would be I would then have to find speakers to match the amp.

i'd find something new for the head unit just because that ones fairly old at this point. its still fine to use obv but given you look at it all the time its always nicer to have a "new" feeling to your stuff now and again especially with the newer and user friendly tech of head units lately.

amp is fine for a placeholder at least for now but i don't see any real reason to replace it if you didn't have any problems with it.