PaaS is probably your best bet if you want to avoid hiring someone to manage things. Even managing two servers can eat up huge portions of your time — especially if these are new waters for you.

PHP/MySQL put you at a little bit of a risk, performance-wise, so be sure to keep an eye on resource usage.

In my current role, I'm putting WordPress sites up against thousands of concurrent users, so while the exact usage probably isn't the same, the tech stack is similar.

I'd probably do something like this (pretty much what I use): a Rackspace setup that can automatically scale when traffic spikes, with New Relic to keep an eye on performance and leaks in the app itself, and probably nginx/Varnish to try and keep the load off the servers in the first place (assuming any part of your app is cacheable).

A stack like that is not cheap compared to free, but it's cheap compared to a full-time ops engineer or spending a Friday/Saturday night trying to self-diagnose bottlenecks.

Good luck!