Initially I took my car to Extreme Limite at Faye de Bretagne near Nantes a couple of times for a check over but more an excuse for a holiday in the Loire rather than for anything specific! It was a confidence building exercise more than anything else and a quick and easy trip over the Channel. Extreme Limite have mechanics than worked on the construction and development of the cars so they know them all inside out. But slowly and surely I've realised that generally the 300 is neither complicated nor overly specialised beyond the expertise of any competent mechanic. (I think Iain has found one in Somerset as I have up here in Worcs.) General servicing ie. the change of oils, plugs, filters, brake pads and belts is a simple task on mine and I carry them out regularly with the help of mechanic friends. Access to the engine with certain jobs is a bit awkward (esp. spark plugs) and probably worse on yours. I've created an inspection hatch behind the drivers seat to access the distributor otherwise its an absolute bar steward to get at! I'm not sure how different your Biturbo is from the ordinary 300 in the engine compartment.
Occasionally you will run into a problem that is pure Venturi and this requires a bit more ingenuity and enterprise. ie. recently a joint on my front wishbones had corroded. Cost to buy new from Venturi £1300!! ... Local engineer refurbed inc. inserting some new metal... a perfect job for £200 and now better than the original with no hidden water trap!.
The car's alarm did cause problems a few years back. A new alarm system from Pauly solved the problem. The stiff gear change was isolated to a problem with the cables. These were copied and replaced by Speedy Cables thanks to help/advice from G'ski.
Its a matter of finding competent people around you and I have to say this takes a bit of time but the Club will help in this regard.
That said I've got a squeak in my rear hub that isn't brake, wheel or bearing related... and that I haven't solved ...yet!
A110 Berlinette. Venturi Atlantique 300