Vintage Road Bike vs New One to Amsterdam

If you've got £600 floating around, that will get you something fairly modern and well spec'd on the second hand market. If you don't want to keep it after the ride then sell it on and probably get your money back. Simples

hell if you've got £600 to spend I'd wait till you get to Holland buy some classic bike from a dutch bike shop at 75% of the prices you'd buy for here in london / uk and ride it home and make a profit
that would sadly mean riding fixed to holland and ghost riding something back for 700 miles
not the easiest option !