How important is it to have a level shower tray that is to be used with an enclosure? How 'level' do I need to get the tray?

The more level the tray, the easier it will be to fit the enclosure and the better the water tight fit will be.

As long as the tray is stable a level tolerance of 2.5mm is usually acceptable, with a good quality spirit level this accuracy is easy to achieve. Please refer to the installation manual to check the out of level tolerance, or maximum adjustment for your shower enclosure though to ensure the best fit.