Loads of people (mainly if they’re new), ask this question, and the answers are unfortunately more questions:
What do you want to do?
How often are you going to use it?
How will it work with other gear?
It’s quite easy to get sucked into the idea of buying things (ooh, nice shiny lovely things) but a lot of the time, unless you’re using whatever it is on a frequent basis, you’re probably going to lose money on it.
Have a look at a local rental company (for example, in London there’s Richmond Film Services, The Audio Dept and Better Sound to name a few) and see how much it costs to hire whatever you need. Also it often costs only slightly more to hire professional level gear (Audio Ltd, Sound Devices etc), over consumer gear (zoom, sennheiser evolution) while the professional level gear will be more robust, sound better and be able to interface with other professional gear more easily.
Also look at the features in gear that work for you personally and get the gear that works best for the way you find you work more often. There’s a few things that can catch you out around returns/routing on certain mixers which you sometimes expect to be there and sometimes aren’t (like where the return signal can be routed, is there a ‘bus in’?)
Also, always factor in the cost of cables when you buy something, you often need quite a few new ones with obscure connectors on