The lowest risk is to use the Top 1 EA. As to how long it will take to pass the challenge depends upon the volatility of the market. There are certain days, weeks, and months that are more volatile than others.
For a newbie, I would recommend using a 10K challenge. This way if you fail the challenge you can use the same money you would have used on the 25K or 100K challenge to try the 10K over again until you pass then challenge. Then once you pass the challenge you can use profits from it to use higher level challenges.
Alan,