Yeah, I’d 2nd this as well generally just for the compressor as much (likely more so) than strictly the ‘flavor’. That said, Heat is not a ‘bad’ choice, but generally not the only or best master bus tool. Keep in mind, hardware like the Warm Audio is going to be subtle… it’s the last 5% of what you’re likely looking for. You’re asking about external gear, but is that a strict requirement?
Personally, I’d go with plugins to start which would get you 95% there for less money and allow you to experiment more and learn your overall preferences before buying hardware. And that’s coming from someone who very much likes outboard hardware.
Here’s some plugin recommendations:
Sonnox Oxford Inflator – stalwart and long time choice, but for good reason. It just makes things (usually anything) sound better.
Fielding Reviver – don’t let the $29 cost fool you. Tons of vibe and flavor.
Plugin Alliance Vertigo VSM-3 – from mid/side subtle to destroyer of worlds and everything in between.
PSP VintageWarmer2 – another classic, but still often used here.
And there’s a bunch of others depending on the vibe/flavor you’re after… SoundToys Decapitator, Kush Audio Omega’s, Plugin Alliance HG-2, Fabfilter Saturn and the list goes on.