Geoff,

The second point is easier to answer than the first, I guess the order of performance of endpoints would be something like -

1. Ultra DAC
2. 818v3/808v6
3. 818v2/808v5
4. 861v8

Etc., don't know how far back you need to go or if you intend to run DSPs or analogue speakers.

I'm guessing you don't see the former point discussed very often because who knows when a particular model will be updated, apart from Meridian themselves? Again though, at a guess, I'd say you'd be OK with 818v3/808v6 right now, but I'd put money on 861 being updated sooner rather than later, and some of the G series and MS series getting MQA compatibility soon too.

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502