Buying a brand new Mac isn't always the only viable option. If your current machine has a decent CPU, 8GB+ RAM, and Thunderbolt 3 then buying an eGPU may be a cheaper alternative. For more information on recommended GPUs please check the following article from Apple's support page: https://support.apple.com/en-us/HT208544.
Be sure to avoid Nvidia cards, Apple chose not to sign their drivers and therefore these GPUs will not work on macOS Mojave and later.
If you'd like to buy a new iMac or MacBook Pro instead, then, in general, any machine with a dedicated graphics card should provide good performance. That said, keep in mind that the weaker the GPU card you pick, the slower Motion and FCP (and our products used inside them) will run.
Finally, bear in mind that graphic cards (even discrete ones) can not be upgraded at a later point, so you might want to consider buying a Mac with a powerful CPU and graphics card, but with less RAM and/or HDD space as the latter ones can usually (but not always!) be upgraded further down the road.