I think a garage specialising in just Renault Turbos might be hard to come by, I doubt they would get enough work to live off. There are a few places out there that specialise in the R5 but your unlikely to find one in every town.
If it's bodywork, suspension etc then any half decent garage should be able to give it the once over and tell you if there's anything obviously wrong with it. If it's engine tuning your looking for then somewhere that knows how to work with carb engines and has access to a rolling road would be best.
Ideally though, especially for saving on labour charges it would be a good idea to think about learning to do it yourself, maybe some friends with more knowledge/experience could be payed with beer tokens. :mrgreen: