Definitely need a new Canbus Upgrade otherwise you experience battery drain with a 2004 model. You need at least a Revision L Canbus Gateway but any 7N0 907 530 module will work fine with the MK5 Golf but will need coding with VCDS (VAGCOM).
Installation of the stereo unit is quite straightforward, make sure you got a T20 Torx screwdriver to undo the bolts of the vents and bezels. A pry tool is handy on prying the bezel off, from there it's just a matter of unplugging the Quad Lock and swapping the new unit in. You most likely need an fakra aerial adapter wire from the old stereo to the new one too.
Bear in mind the RCD510 does not come with Bluetooth as standard, you will need to purchase this separately if you want telephone functions and will require installation from under the seat to the stereo (as well the mic in the welcome light section).
Alternative OEM stereos can be viewed on this thread
here for your reference.