I wouldn't be handing over any cash. This thing smells like a scam so far.
First off it's a boat brokerage in Salcombe, but the actual boat is at the other end of the country in a warehouse?
Zodiac 500s aren't particularly rare and with £12k to spend you'll easily be able to pick a great example. First off, most are PVC, so I'd want to visually inspect tube-set for wear and tear. They're damned expensive to replace otherwise.
Hull and fibreglass deck fittings need a thorough inspection too. Early examples used to get spider-web gel-coat stress marks. Serial number on hull ID should match with original delivery paperwork. It's only 2012, so that should be available.
Lastly the engine. I'd want more paperwork than the UK's Brexit negotiations, compression test across all cylinders and to see it run from cold on a frosty morning on muffs.
I'm free at the weekend if it's local and can post pictures, but do your research first.