Might need a 64gb sd card. Does it matter what brand? Sandisk ones are pricey compared to ebay stuff. Does it really make a difference?

Well yes; eBay* stuff carries no guarantee that the capacity shown is actually what you get. Many, many fakes out there.
Caviat emptor, buy from someone reputable, like 7dayshop, MyMemory, or someone like that, and you absolutely get what you pay for, especially write speed.
Kingston and Transcom are ok, I’ve got a bunch of unbranded 8Gb cards from 7dayshop, Toshiba made, four for fifteen quid, IIRC, on a special offer, and they’ve been excellent.
*Amazon can be dodgy, too.

You get fakes on ebay sometimes that are really slow.

I have always gone for sandisk and have never been disappointed.

Dx.com

Sainsbury’s keep doing all memory at half price.

7day shop and Mymemory are some of the more reliable cheap card vendors, wouldn’t buy any off eBay, too many rebranded fakes.