I've been doing that research for my M59, and now have a towbar and electric kit in my boot awaiting install. In basic terms, I would think the same as I found would apply to newer Berlingos too:
On the M59, the back of the car already has bolt-holes in it for attaching a towbar, and the back bumper already has two small holes in its underside, to allow a small rectangular cutout to be made in the right spot for the towing hook to poke out of.
I found this out by downloading the fitting instructions as pdfs from 2 or 3 towbar suppliers.
As for the electrics, first you need to know how many pins you want in your socket - either 7 or 13. 7 is fine for trailers and more, 13 is a new euro standard best for caravans etc. But it doesn't really matter because you can also get 7-13 pin adapters.
Next you also need to know if you need a bypass relay or not. The M59 doesn't need one because it has no bulb failure detection gizmo. Therefore the trailer lights can just be spliced into the car's rear light wiring. I would guess that the K9 does have bulb failure tech, in which case a bypass relay connects to the car's rear lights but only draws minimal power so as not to set off the bulb failure warning. The relay then feeds power direct from the car batt (via a fuse) to the trailer lights. So a kit with relay also has a long cable to run from the boot to the car batt in the engine bay.
Once you have sorted these two options, then you can decide between either a universal wiring kit or a dedicated one. Either will come with 7 or 13 pin socket and relay or no relay as appropriate.
The dedicated one costs a lot more and you must get one to suit your actual car, but the upside is that it will just be 'plug and play' with no hassle.
Unfortunately for me, not one supplier stocks dedicated kits for the M59 any more - it's too old.
The universal kit is basically a long multicore wiring loom and other bits, with some Scotchlock connectors. You will have to figure out which wire to splice to which wire in your car etc, but these kits are a lot cheaper.
Several companies supply all you will need. I started with
https://www.pfjones.co.uk/ because someone else mentioned them here some years ago, but they messed me about and I had to cancel an order. You can download installation pdfs from their site.
I ended up going with
http://www.towbarsdirect.co.uk/ - they could not be more helpful, listing all the bits I would need and their prices, and even attaching the installation instructions to their reply as well.
HTH.
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