Your going to get alot of recomendations, and with a 10k budget, you will get what your after. That is a sensible budget for a 500bhp + Engine. What you need to take account of though, is the ancillaries your going to need to keep your engine reliable, but for 10k I think it'll all be within your budget.
Obviously everyone will recomend their engine builder, which is only to be expected.
I would thoroughly recomend Karl Norris (NMS MotorSport) as it was Karl who built my engine. His knowledge of the Cosworth Engine can only be described as One of the Very Best in the business. What you must also remember, is that it will be Karl who does absolutely everything on your engine..... Head Modifications, Measurements, Machining, and Mapping, and personally I think this is a huge advantage of alot of tuners.
Give him a call, as I know he will advise you very accurately as to what he will use, how he will do it, and what to expect.
Anyone (Relatively Speaking) can build an engine that will produce 500bhp, but making it reliable, making it drive the way "you" want it to drive is a major input into what components to use. Explain exactly what you intend to use the engine for....Road, Track, Race, Rally, and he will match the components for your application.
Just give him a call, and the chance to explain what you want, and what he can provide... You won't regret it.
Check out his web site......
http://www.norrismotorsport.co.uk