If you want to be as safe as possible I would suggest you get: -
An MCA bill of sale (transferring ownership to you)
A proper contract (explaining what is included in the sale (warps, fenders, nav equip, etc)
Something that shows vat has been paid at some point in the past
An engineers report on the engine (normally less than £50 to plug it in to a computer and download)
Proof of services, repairs, etc
Checking the HIN and engine number (with the engine manufacturer) is also a good idea.
You get what you settle for!