Hi Rupert! As J.J says get the mechanical side of it thoroughly checked out then based on what you find out about that,make your decision on the bodywork.I go to these guys for servicing and repairs
http://www.pecarspecialists.co.uk near Barnsley,they are very good!

From what I read on here £1000 a side is about right for a THOROUGH sill replacement job.A TIPEC member in Scotland makes sills for these so you may be able to save a bit by getting them from him.Lee 944 will point you in that direction when he reads this.Considering what you have already paid and the fact that belts, tensioners and other jobs have already been done,I would probably carry on and get the bodywork done too BUT be aware that you will not get your money back should you decide to sell,you should however have a very nice car!

Good luck with it! John
