Should I buy DDR5 RAM now for a future build?

No. Wait until you actually build your PC to buy the RAM. Prices will keep dropping, and you’ll get a better deal with current-gen stuff when you’re ready to buy everything at once. If you buy now, you’re locking in today’s price for something that’ll be cheaper in six months, and compatibility or revision changes could cause issues. RAM is not a collectible—buy what you need when you need it.