@Simon Dale cordless tools aren't the thing to scrimp on imo because you're just wasting your money. Go for as expensive as you can afford because the difference in performance and quality is really evident.
Hilti is the best imo then the next best brand for cordless for much less money is Metabo before you drop down to the likes of Dewalt or Makita etc. There's loads of brands out there but as already said go for a brand that offers the best compatibility with a large range of tools.
I have a Metabo cordless grinder and it's been put through hell and is still working like the day I bought it.