The Deck SD slot has UHS-I support so maximum read speed is 104 MB/s even if your card is faster. So no need to get any top-class extreme card, as long as it's UHS-I it should be good to go.
It cost me $30 to add 256gb of storage in the form of a microSD card.
I’m very happy with it and I think I got 95% of the satisfaction with minimal cost. There are some moments when I wish I had the better screen or the dock, but not really.
Its been great.
I'm really happy with the antiglare screen. I can't always play in the dark.
It is possible to replace the SSD although i am pretty sure it voids the warranty. Valve's recommendation is that you add an SD card, which is a lot more reasonable these days.
Sorry my answer has been kinda rambling yet non descript. tl;dr i don't think storage is the biggest concern, but the anti-glare glass is great.