caraudiosecurity normally have some good deals – halfords *sometimes* do, check prices on internet against instore prices.
Avoid ebay – lots of grey import/ dodgu stuff on there.
If it’s just a head unit – should be pretty easy to fit – may need an adaptor (about a tenner)
If there is an in car remote / display (renault haves a seperate screen on the dash for what radio station is playing) this will be a more costly adaptor – or it will stop working.
Head unit and replace the front speakers will be better.
I spent £100s on a proper install once (nine speakers, 2 amps, sub, power cap, active crossovers etc.) my mate spent £100 on a sony head unit and £70 on some kenwood coax speakers – was a lot better sounding than mine!