EBay is good for towbars , not sure on the best brand though.
Towbar is just bolt on.
Electrics is just a simple matter of tapping into the rear light cluster.
As long as you don't have multiplex wiring of course.
If your confident with electrics , its a satisfying job to do yourself
I got mine off ebay, the only problem I had was that there are a couple of spacers that go inside the chassis rail to stop it crushing. It is easy to drop these inside if you don't think about what you are doing. Other than that it used factory holes, no cutting or drilling required, but I had to remove the rear bumper to fit it and wire it in. Wiring on Citroens is not the easiest to decipher. I took all the wiring from the N/S cluster other than the O/S rear light which meant one wire running behind the bumper.
As has been mentioned, Ebay is the place. It was a very easy job to put the towbar on my MK2 - first time. However, there were a few stories about the wiring which scared me off but I managed to pickup the original Citroen Berlingo wiring loom for towbar and trailers from Ebay also - it was only £20 and this went in fine. I'm sure this was overkill and there's a simple way to sort the wiring out though.