If you can up your budget just a touch, then you could get a nearly new Denon X1800H for £314 (2 year warranty)
LINK and also a pair of used Dali Zensor 5 floorstanding speakers
LINK for £150 which will sound great and go down to about 43Hz, so whilst not subwoofer vibrating levels it will still give some deep audio and can create a phantom centre.
If you want a centre further down the line, then Dali Zensor Vokal is a good speaker and can be had cheap at less than £100
LINK
When you said you are well outside your wheelhouse, just how outside are you meaning? I was brand new to the whole AVR thing last year, but it turned out pretty straight forward and I'm glad I jumped in. The Denon has a supplied microphone you connect and it auto calibrates everything.