When you get YOUR survey done make sure you discuss with your surveyor how much of a survey he will carry out on the engines and gearboxes.
In many cases it will be minimal. You may have to employ a seperate propulsion systems surveyor.
N.B. In the UK older boats of the type you describe with twin petrol engines are almost unsaleable. If you can afford the running costs you buy something newer.
A single diesel is preferable. Like THIS