I'm with you Mr McGoo, sorry McGraw. "If it ain't broke, why try to fix it?"
The easiest way to replace a LR head unit is with a LR head unit. Unfortunately there are a number of different fits in the Disco, base/mid line which doesn't have the additional power amplifier, the Hi-line which does have the amp and a CD auto-changer and the Harman Kardon "deluxe" fit which has everything which the Hi-line has but has HK speakers instead of "ordinary" ones and the built-in sat nav. the Hi-line and the HK fit are sometimes confused.
I don't think I'll ever understand those people who buy a Disco and instantly start turning it into something else, likewise the immediate need to rip out the original fit then start asking questions like " I've got this huge bundle of wires and nothing works, how do I put this ten kilowatt drum and bass speaker under the passengers seat?"
The OP (iamthewelshtaffy) asked if there was a head unit with a CD player which would plug straight in and I gave him the part number for the LR mid line fit which does have a single CD player, but others then jump in with advice which might well be valid, about fitting this or that amplifier with or without various adapters and huge bunches of wire.
Questions about ICE elicit some of the most complicated replies sometimes.
As I've said in the past, we're talking about a few sounds in a car, a finely tuned audio theatre it certainly ain't.