Although this listing is for a 8 cyl conversion kit the contact details and bulk of the info are the same.
http://cgi.ebay.co.uk/MOTOGAS-AC-8-CYLINDER-LPG-INJECTION-GAS-CONVERSION-KIT_W0QQitemZ330234758790QQihZ014QQcategoryZ36631QQcmdZViewItemBear with Jeremy, he's very busy at the moment and may take a while to get back to you, but he will!
As for your questions.
1. As Tunnie said, depends on mileage and how long you plan to keep the car. You should be able to save around 8p per mile... I do an average of 2k miles per month so I'll save somewhere around £160/month... Lets take a worst case scenario and say I save only £100/month... It still pays for itself in under a year!
2. Again, Tunnie's right... You will lose a little of the economy but not much!
3. I'll let you know!!! I am collecting my kit from Jeremy in about 2 weeks time and I'm hoping to get everything installed and running ASAP, realistically it'll probably be cooking on gas by the end of August due to other commitments!
The decision is very individual... My last car was LPG and I said I would have to think long and hard about another conversion but at the price of these kits, which are proven to work well on the Mig, I can't really go wrong!