So I looked at going down the apple play route but looks expensive. So I currently have the standard gs spec 08 plate freelander with the stock 4000 head unit. I can get the next model up genuine stereo which is the Bluetooth version. Will this just seal straight in and work or what else will I need. I know I will need the right bit on steering wheel that as the phone buttons on. But what else

Do I need to change any wiring
Will I need to source a BT module or are these built into stereo?
Do I need a microphone or again are these already built into the stereo if not how is it wired in etc
Thanks

It won't just plug in and work. The systems are vastly different. The easiest is an aftermarket Bluetooth phone kit like a parrot. If you are after better quality sound then replace the speakers with the ones from the higher spec audio system. Makes a huge difference to the quality of sound.