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Buying a completely 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 become a much cheaper alternative. For more information on the recommended GPU cases, 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.
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 being said, keep in mind that that the weaker graphics card you'll pick, the slower Motion 5 and FCPX (and our products used inside them) will run.
Lastly, remember that graphics 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.