What should i spend my coins on?

I was also thinking the slates but 40 coins for a 10% chance seems like such a waste for me. I'll probably get like 30 2h 50%s to finish my heart and scroll something else eventually and invest the rest into 20%s 2h wep atks for another sword if i ever find one again or my fafnir polearm for my Dark Knight. do the 20%s expire in 3 months like the 50%s?