The speakers are well regarded... and speaker technology moves slowly (top speakers of the 1980's are still top speakers today!).
Buying used for speakers can make a lot of sense as a result... good value to be had there.
Buying used for the AVR can be more of a question mark.
The AVR technology has progressed quite a lot... current AVR's have much more sophisticated Room Equalisation software (Audyssey, Dirac), also things have moved on from 1080p and HDMI 1.4, with all the recent AVR's supporting HDMI 2.1, and up to 8k displays, with the new high contrast/colour gamut standards (Dolby Vision, HDR etc...).
In terms of the audio side of things, the AVR's of 10 years ago were primarily "base layer" with 7.1 being the standard - now we are talking 7.1.4 being more the norm, with the addition of height speakers, and the introduction of Dolby Atmos, DTS-X and Auro3D all providing the additional decoding required to feed the height channels.
So you will have to consider where you want to end up and how you want to get there.
If you are happy with the 5.1 speaker setup vintage 1990's - the HK 5500 is a good AVR... but it will not take, or pass through the latest audio/video standards.
So I would seriously consider a new AVR, and probably an appropriate external power amp, to accompany those speakers.
Why an external power amp? - most of the "budget" or mass market AVR's just don't have enough current, a beefy enough power supply, to drive difficult 4 ohm speakers like these.
I have similar specced speakers - and I run an Integra DRX 3.4, feed the Main L/R channels through an external power amp, and then use the internal power amps to drive surround channels and height speakers. (I run 5.1.4) - The AVR cost me around AU$1750, and the amp cost me around AU$400 used, including shipping, I brought it in from USA, after checking it was compatible with 230/240V.
Other alternatives are AVR's such as the Onkyo RZ70 / Integra 8.4 - these would not require an external power amp, as they have powerful amps with substantial power supplies - but they also cost AU$5000
(they are also capable of running 7.2.4 - and some of the latest Room EQ software Dirac DLBC, where my more basic Integra 3.4 can only run Dirac Live)
Note: I find 5 speakers perfectly adequate for the base layer, adding the 4 height speakers definitely takes things up a notch - in a perfect world you want them all to be timbrally matched, but in reality, you can adjust for that with software like Dirac - by setting a target EQ curve that matches the base layer.
Yes we definitely end up paying an "aussie tax" for the priviledge of participation... gear is more expensive here than in the USA.
P.S. I am making no comment about whether the speakers are good value or not, as I have not been tracking the B&W range for many years....
But you can look at current new equivalents to get an idea of where pricing is at.
And do keep in mind, that as long as the drivers are in good undamaged condition (including surrounds, tweeters not "poked" by kids, etc... - good speakers from 20 years ago are still good speakers today)