I would go for the Raylar HO 525/600. After you do this proven upgrade you will have a solid, turn key 600 HP engine. The advantage of going this route is you have a zero hour engine again. There is nothing wrong with blue engines when they are new but most of the low hour used I imagine are have been rode pretty hard. No telling what you might be getting into. I have always been told that going cheap will be expensive. Either do a complete rebuild with the Raylar or new Blue!!