I looked in to buying one a while back but decided against it for a variety of reasons.
Are you entitled to drive vehicles up to 7tonne? If you are restricted to 3.5 tonne it will be difficult to keep within the weight restrictions, especially when fully loaded - the driver and passengers count too, as well as bottled gas, water, and so on.
Do you have somewhere to park it when you are not using it? This can be a problem.
I wanted to drive across Europe - and was worried about the 'hijacks' that were frequent at the time because it is obvious that your passports, money and so on are on board. I believe the situation hasn't changed.
And the practicalities. You have parked up for the night, about to dish up dinner, when you realise you have no milk - it's not easy to drive to the nearest shop five miles down away.
There are places in the UK where you can hire motorhomes for your holiday. I strongly suggest you spend a few hundred pounds and have a test holiday before committing many thousands.
This is a handy site, with an excellent forum of keen motorhomers:
http://www.practicalmotorhome.com/news/index.h tml