I am in a similar situation.
Although I am trying to set up a garage of some form, a renault specialist seems like a good idea to me. Don't worry I may degrade myself and work on Vauxhalls! Only kidding
At the moment I get an occasional phone call to lower a torsion bar or fix something wrong with a motor I would get a lot more if I had proper premises which would allow me to get more tools.
The SBG site that Scotty posted a link to is quite good. Thats who I have to go and see. I am hoping the princes trust is gonna feel sorry for me and gimme a helping hand, otherwise I have no chance (4 quid over 2 bank accounts! lol).
Becoming a mobile valet ain't hard. Get a van, write along the side 'stereo rob mobile valeting' (or whatever) and drive about people will notice and you can get annoying and print off ads on the pc and stick them under window wipers. I know people who have done this sort of thing and some have become pretty successful.
Main costs are the materials to start up, the mobile petrol power washers for example with their own tank start around 500 quid, then theres yer sponges, chamis, polishes and degreasers and stuff.
A good place to get business is industrial estates etc. You can offer the full works or just a quick wash, theres a guy at the local Tesco who does a quick wash, has done for years, even in the pissing rains hes there and theres always a Q of cars waiting for a wash.
Good luck and sorry for the long post!
Chris