Everything i have highlighted in red rings alarm bells in my head, the blue is the contradictions that ring even more.
Its never been registerd but it has insurance and is on the dvla website, so it must have documents. The insurance company web sites say it doesent exist, that could cost them business so i would be cautious again. As to the new engine pah for £450, it won't be a new motor it will be a 'rebuilt' or another motor at best, not new.
It stinks, really it does. Do what you can to get out of the deal, give him a deadline or claim missrepresentation, hell just tell him your skint and tyrekicking, and shop properly with someone who's opinion on M/C's you trust.
A bad first bike is the worst, like a bad first date.