I (as a beginner-level whistle player) say:
buy a cheap one in D
Most tunes are either in D or G which can both be played on a D whistle.
Cheap whistles come - in most cases - with beginner's instructions explaining which note requires which fingering.

A cheap one is good enough for the first steps - and if you decide it is not "your" instrument you didn't make a too big mis-investment.

Once you are sure you will go on whistling there is still time to look for more expensive alternatives.