Threshold made fabulous gear, but a 400A would be >40 years old now. If it hasn't been updated, you should definitely include the cost of having it gone through, and at the very least having the power supply & other electrolytic caps replaced.
It will still be less expensive than a Pass or First Watt amp, but you get great sound at a discount. If you have the budget, Pass is still going to be your safest bet, though.
Nelson has always been clear that he knows we all have different sound priorities. He started FW as a venue to experiment with different output devices, and optimize their unique sound signatures. That's why FW doesn't have the traditional pricing hierarchy found at most amp makers. While a fascinating concept, it puts more of an onus on the buyer to choose their own favorite "sound".