You can never be completely prepared for programming interviews. How well you do in a particular programming interview depends on whether you have had any experience working on similar questions/problems in the past.
So, bottom line - program as often as you can.
In the project description, mention that "only people who will provide a sample solution/data for the project will be entertained". This is a decent filter to remove 90% of applicants.
To decide between the rest 10%, check ratings, previous jobs, portfolio, cost etc and make a choice.