Tow bar I can help you with:
I got mine from
http://www.hedley-towbars.co.uk/
Brink 4361
Fitted myself and had no problems.
As with all tow bar fitting the electrics can be a little challenging.
Fiat do a wiring kit:
Westfalia 306 263 300 133
Fiat 50 901 429
However this kit only covers trailer lighting so if you are towing a caravan or other trailer with onboard electrical items and charging systems the you will require additional electrics.
I went the independent route and used/fitted:
PCT ZR1328A - Towing Interface Relay
PCT ZR1220 - The Volton Combi Relay
Both Units are available in kit:
PCT ZK3020A - 12N/12S Towing Interface Relay Kit
Additional point of warning.
Your need to run some fairly heavy wiring directly from the battery to the rear of the car. This easily and best done buy routing externally following the brake and fuel lines through the plastic trunking under the car and end entering the boot from underneath at the rear of the car.
Personally I ran two power feeds, both fused at battery end. One for the lighting circuits and one for the auxilary power (caravan battery charging & fridge etc.)
When wiring into the lighting circuit I used quick connect insulation displacement connectors (also know as ScotchLock) but a note of warning. You MUST use a control unit as detailed above (or similar) AND PLEASE DO chase the wire back from the rear light units AT LEAST 12inches/30cm from the light unit themselves. To do this you will have to undo the cable loom rapping/tape but the idea is to ONLY connect into the loom where you can easily patch and repair poor/broken connections easily. I've seen so called professional fitters tapping into the looms right nect to the connectors that go to the light units.
LOOM ENDS/CONNECTORS and wiring must be protected at all costs. Always work well away from the ends and certainly far enough away to allow several repairs to be implemented before running out of useable wire at loom/connector ends.