What should I look for in a virtual office?

There are so many virtual office formats. Sales folks usually work out of their cars and Starbucks. All business is transacted at the client site. But the most formal virtual office I experienced was at a shared office location. The provider was a company called Regis (absolutely no connection). It was great as all services were available on an as you needed basis, I.e. reception, conference room, mail service. The car/Starbucks or work from home structure becomes a little troublesome when location licensing is involved. I am sorry I do not recall the costs, but I know it was at that time a reasonable option. I came and went as I pleased with a locked door. Interestingly, the internet was hard wired, for security reasons.
Goodluck.

Cowork space is useful depending upon the specific needs. I have worked in one in NYC, a permanent desk was $500 per month, that gave you access to a secured room with other permanent desks, copying/printing (with a limit per month), and conf rooms. It was startup focused, so the collaborative nature was free. No phones though. A roaming seat in the community room was $300 per month.