When should I pay for a mechanical inspection on a car I'm looking to buy?

I am looking to buy a used car, and am unsure about when, during the process, is best to pay for an independent mechanical inspection (such as offered by the RAC or AA). Of course, I would like to only...

What should we look for when buying a used 1997 Toyota Previa?

I am considering purchasing a 1997 Toyota Previa for my daughter to enable her to take out her five children at the same time. The vehicle has 147,000 miles on the clock but seems in pretty good condition....

I want to buy a used Porsche Boxster - should I have it checked by a Porsche mechanic or the AA?

I'm looking to buy a five-year-old Porsche Boxster from an independent motor trader. Would you recommend the vehicle be checked by an approved Porsche mechanic or have an AA inspection?