James Spader has the experience and the acting chops to pull off Ultron. The only difficulty is his age, but CGI technology can alter Spader's looks to make him younger or put him in whatever form the director wishes. They can even put him in performance-capture mode similar to "The Polar Express." Spader will do just fine and has the emotional depth to pull off an evil super robot bent on destroying the Earth.
I do not believe that James Spader should be attached to the Ultron role. Pretty much every role I've ever seen James Spader play.. he has only played himself. It's always the same droll voice and dry mannerisms. Being a superhero film I believe somebody even larger in personality should be cast.