Hi Gordon
Welcome to the forum
I don't know what the towing limits are for your car so make sure that you know what caravan if any you will be able to legally tow before spending any money on towbars etc.
Copy and paste the text below into your web browser or gain access via the Home Page of this website (clickable links there) to information about towing weights.
http://www.practicalcaravan.com/legal/index.html
Detachable towbars are perfectly safe, the frame of the towbar is fixed permanently and the towball is detachable via a locking mechanism, I've never heard of one failing yet. The advantages of this system are that when not in use a flap covers the towbar gubbins which looks better and enchances the resale value of your car
Before you determine what towbar to fit you need to note a few facts about the caravan that you want to go for.
Most modern caravans have an Al Ko stabiliser fitted to the hitch which is why your fitter asked if you want a long or short towball.
Al Ko's use a towball with a slightly elongated neck so unless you know how old your caravan will be I'd ask the fitter to leave the actual towball off for now.
As for single or twin sockets -
Cars which are fitted for towing caravans will also be fitted with at least one, if not two electrical connection sockets attached to or adjacent to the tow ball. Commonly known as 12N and 12S sockets. Power from the vehicles indicators, fog lights and sidelights is transferred to the caravan lights of the same type through the 12N socket, whereas the supplemental power required by the caravans reversing lights, battery charger and fridge are sourced through the 12S socket. If you are fitting your own 12S socket you need to remember to fit a split charge relay to your car as well, this ensures that power to these high drain devices on your caravan only flows when the ignition is turned on. Should you not fit a relay of this type to your car and take a break during a long tow, you are likely to find that your car battery will not have enough power to start the engine when you resume your journey.
New caravans are mainly fitted with the European 13 pin socket which are considered to be superior to twin sockets.This means that if you bought a new caravan you would need the 13 pin socket on your towbar.