hi Nick, welcome to the ribbing world!
the ONLY thing that matters is you are happy with your purchase so keep that in mind.....
the engine is fine in theory and the hours are not an issue if serviced as you describe, obviously get a service yourself and if the timing belt (it may have a chain? hopefully one of the forum yam gurus will know or a google will confirm) wasn't done i would get it done before you use it. best speak to your local yam dealer for info on this one. they can sort out an oil/filter and gearbox change too if you don't fancy it yourself.
the tubes are starting to fail going on your video so you have few choices, you either live with it whilst you budget a retube (5k ish) or ignore/paint them as you suggest.
It is not normal for ribeye to have hypalon, not impossible though so you can tell by doing this- http://www.ribstore.co.uk/cms-assets...r-inflatable-b
what you are describing sounds like hypalon though so that link will confirm.
painting tubes instantly devalues a boat but probably no worse than tubes failing anyway. if it were mine i would probably use as is, if the tubes don't leak! and get a feel for it and if no leaks get painting it.
the trailer looks fine, new brakes and bearings will cost less than 150 quid or so worst case. one thing though is the rear swing beam needs moved back closer to the transom for me.
hope that helps....enjoy!