The M3 is a little long in the tooth but it was certainly very well received in 2007 when reviewed by Stereophile
https://www.stereophile.com/integratedamps/107nad/index.html
Being part of NAD's Master Series it was built expensively and a big step up from their other amps. If it's in first class condition, it will give you good sound and probably many more years of service. NAD has moved to Class D now but wisely avoided the early Class D efforts such as Tripath. The M3 (as far as I know) is a traditional AB amplifier. It has all the analogue facilities you're likely to need, but no digital, so no DAC.
To estimate value, take a look at Audio Shark and see how much others are being sold for. Personaaly, because you still need (probably) a streamer and DAC, I'd put the money towards a good used NAD M32 if you can find one with streamer module included, or (at lower quality but including room correction) an M10. The M32 is discontinued and replaced by the M33, but it's a first class amp and unless you have mega-bucks speakers, you'd be hard pressed to notice the difference between M32 and M33.