As the car has service history missing i would phone the last dealer who serviced it and ask if he has done any since ,could be the book just did not get stamped ,you also need to rule out that it has not been clocked as these cars will do big miles and not show it ,so ask the last garage who serviced it for as much info as possible .check steering wheel ,brake pedal, carpets(including lifting overmats) for wear.look at what dealer name is written on the number plates and give them a ring for info and as a last resort phone previous owner from the log book to check mileage .most important HPi the car even if the dealer tells you he has already done it ,it will verify mileage and tell you of any accident damage etc.ask them to knock the price of the cambelt and service of the car because at that mileage if it has been serviced the cambelt would have been done already .and to be honest i would rather get some money of and give the car to a garage i know and trust to service it especialy when a cambelt is involved .price sounds about right for the gs300 as the mileage is low but stick to yours guns and he will fold .