I can only speak from my experience, but last summer I bought a clean '91 27' Fountain (most poeple accuse it of being 25') for $20,000. It had a base 300hp dog motor, but will handle anything I want to throw at it. I've since put in a 630hp 555.
Just take your time, and the right deal will come along. I didn't modify my last boat since I didn't feel that the hull was designed for big power or speed. If you buy a solid base, you'll never regret it.