I have been considering buying the Mk2.5 rear lights, also thinking about changing the front headlights with the Black inner ones.
I wouldn't say the Mk2.5 bumper was a straight swap the reverse and fog lights have different connectors to the MK2 so you have to buy the mk2.5 reverse and fog light connectors which are rare.
However I did it different, the bumper that I have still had the reverse and fog lights left in them with the connectors because the wire had just been cut upon removal.
So I cut the wire that was attached to the mk2 connectors, crimped the wire coming out of the mk2.5 reverse and fog lights and then did the same to the wire coming from my car - added connectors to the ends of the wires.... then connected the to wires together and wrapped them with insulated tape.
I wouldn't worry about what colour you buy as your most likely going to have to get it re-sprayed anyway as car breakers tend to store them badly... so it will most likely have scratches and scuffs on.
Check the attached picture I quickly found on the internet for plastic seals...
1539801 & 1539802
took a while to find these Finus numbers and luckily I still have them stored on my phone!
These parts will help with the water running down the sides as it seals the gap you have without them. It also prevents the bonding on the rear window from weathering so much... however I was to late for that but to prevent mine from getting any worse I installed them anyway.
Also FYI... you don't have to take the spoiler off to install them they literarily just sit at the edge/under it.