If you looking for someone who only has a 4* rating from this site then you will be missing out on the hundreds of more than capable VA's out there that do not rely soley on PPH for their business hence no stars.
If you are going to do that you may as well just invite to bid only.
The Case Study is a short task that is a taste of what the full Job will be like and
gives you the opportunity to show the Buyers your skills.
Keep in mind that you have more chances to win a Job with Case Study as fewer Sellers will send a proposal.
Moreover the Buyer has already paid a fee for it, which shows that they seriously intend to award the Job.