If it's not a rude question, how much are they asking for it? 26,000 isn't massive, but I'd think the price should be commensurate.
Pete has done 80,000 on his on the flat tappets and it hasn't shit itself yet.
'After market air box' is a worry. It's not some sort of pod arrangement or similar? I would be finding out what kind of remap has been done to ensure it's not over-fueling.