I think it also depends on what you're looking for that agent to provide. These are the items most of the first time STR buyers need help with:
1. Analyzing local deals for cash flow
2. Knowing the regulations in the area
3. Making connections with vendors, cleaners and repair folks
4. Leveraging resources like lenders, title companies and insurance contacts that have experience working investor clients.
The more experienced investors I have worked with already have most systems in place and use me more for transaction management. I also enjoy the negotiations to extract as much value for the client as possible. You wouldn't believe the number of agents that fail to think creatively to problem solve transactions, then let the deal blowup.
Overall I would say an agent that can feel out what the client needs mosts and provides it to the client. Then also being able to negotiate deals beyond the standard trend of no seller concessions like we have seen in the past couple years.