best 64-bit MS Office for 2012 MacBook Pro?

I have an 2012 MacBook Pro running macOS Mohave 10.14.1, 2.5 GHz Intel Core i5, 4 GB 1600 MHz DDR3. I keep getting message from Apple that I will need a 64-bit word processing app at some point. Currently have MS Office 2008, which seems to be very slow. Looking at MS Office 2016 and 2019. Is the computer too old to make this purchase worthwhile? Which would be better if both are similar price? How long can I hold off? Thanks for advice!