That depends entirely on what you need to get done, though I will say for a stopgap upgrade from a 2013, I’d guess an i7 would suffice.
The bigger issue is storage. I’d recommend a decent-sized NVMe stick in a Thunderbolt 4 enclosure, formatted APFS. My current portable is a 4TB Samsung 990 Pro in an ACASIS enclosure, and it just plain rips. For in-office work I’m running an 8 bay SSD RAID.
I/O will have more effect on your day-to-day than i7 vs i9, and a fast SSD in a TB4 case will translate nicely to the newer hardware when you move to Apple Silicon.