I went with Infinity components up front & Focal 6x9s in the rear. As Bockers has if you do anything junk the standard headunit, and as a second item replace the front components, then the rears, then think about amps. It depends on how far you want to take it.
It's worth putting some sound deadening in the doors when you replace the 6.5" speakers. I used flashing tape to reduce the panel vibrations.
You can comfortably do the
Head Unit,
front components and
sound deadening for around £170.
IMO spent more on an individual item eg HU, then components, so you are able to build up an excellent system over time if you choose.