I'm Weak... Very Weak

How easy is it to switch rear gear? If I buy a fixed gear I think I'd have to get a few different sized gears so that I can switch them depending on how I feel.

Isn't it possible to put a fixed gear each side of the hub and flip the wheel to use a different gear ratio?

Yes. And if you carry a chain tool and lockring tool you can change sprockets out and about too. Which actually can be sensible if you go down the track and you need the extra gearing.

Edit to add -

You can get way with a fixed-free hub if you only use the free side at the track - no skidding there, y'see.