I like it but it would be too tall for me , looks like a PE swinging arm and maybe PE frame ( different rear brake set up to RM ) Pe rear wheel and as Joan says 125/175 front wheel ( maybe you don't need so much stopping power with the engine braking )
PE 400 frame is a lot stronger than the RM frame ( so if it is a PE frame that's not a bad thing )
I think it is a very cool build and would be competitive in the right hands , its a good price I wouldn't think you could build one to that std for what he's asking