Your best bet is to find a used ARC Dac3. You already have an ARC preamp and I assume you like it (as you should). Thus, an ARC DAC makes total sense and will be an optimal match.
I've had the Benchmark DACs (still have the DAC1 as a headphone amp and previously owned two DAC2 HGC), but you can do a lot better. For the money, they aren't bad, but a used ARC DAC3 will be far better.
Just because Stereophile says Benchmark is "Class A" doesn't mean you're getting a DAC with anywhere near the same performance of other DACs that are truly Class A. Trust me...I have an $11k Lampizator DAC and also had a $3k Wadia DAC and both made the Benchmark seem like Class C.
That being said...you can't overlook the source. A standard laptop will put a ceiling on your performance that is tough to get around. As someone mentioned, you can get things like the Regen or iFi USB 3.0 to help, but you also need something with decent processing power to avoid other pitfalls in computer audio.
Personally, if it's possible to wait, I'd save up for the $2k used budget range. You could get a used DAC3 and/or many other previously $3k DACs that have been superseded....plus some tweaks to your server. In the long run, you'll save money and have something closer to true class A right off the bat.