More thoughts... For that price it needs to be mint/complete and from the photos I'd be thinking...
I'd like the seat to be included, can't see it in pics. They almost never come up second hand and they need the correct knurled nut fittings so not so easy to make your own as you can for the Chinese (Honwave etc) style ones. The fittings alone are £60 or more for the Aerotec.
It looks to have quite a few marks on the tubes and I'd want to see these are just dirt that will wipe off.
The air floor looks filthy and these are a pig to clean once heavily ingrained. I would also deflate and pop out the air floor before buying to check the underside for condition and patches.
The trailer seems ill suited or ill fitted to the boat. The keel support/s seem to be collapsing the floor upwards. Does the trailer support the transom/OB?
Think about the OB size/weight balance. It is as heavy as a 20hp but without the power. In truth it would be better to either have a 20 or a lighter weight 15hp 2-stroke Mariner/Mercury/Yamaha.
None of the above make it a bad outfit... just things to consider when looking.
To be honest though I would have thought you could have done better on Ebay or Gumtree around £2000-£2500 without the trailer which I think you don't need from your question about packed dimensions.
As a comparison think about this one...
http://www.ebay.co.uk/itm/bombard-ae...gAAOSwAO9ZPDke
About the same age with the very popular Mariner 15hp 2-stroke. He's been punting it for a few weeks on Ebay and I reckon the several floor patches are putting folks off... but if you could get it at start price it's £1300 cheaper!