How important is a good rack?

Yes, suspended tables like the AR prefer a light rigid platform.

If your turntable has 3 feet then that should influence your decision. 3 feet are also intrinsically more stable and easier to level.
If you have a dense concrete floor also consider siting the T/T on a separate table or rack/shelf that includes an additional shelf for the phono stage/PSU.
If wall mounting always mount the T/T shelf on an external solid wall made from bricks, concrete etc.
Keep vibration sources such as transformers off the T/t shelf. Try to get some form of isolation between shelves e.g. spikes & cups
Watch the cable dressing at the back of the turntable i.e. don't crush, mix or tension the cables, they should be as free as possible.

A facility to level the shelf i.e. T/T platter, may be essential if the T/T has no levelling feature.
Suspended tables are awkward, you are better off using mini spirit levels in 2 diametrically opposite positions to stop the platter becoming unbalanced.

One way to test the shelf brackets is to hang from them. If they flex under your weight like a springboard they aren't rigid enough. Solid triangular brackets are better for that reason.
The shelf material itself should be light and rigid e.g. wood which is thick enough to be considered rigid (you can also screw spikes into wood for decoupling....)
Ideally that shelf should rest lightly on spikes rather than physically connected to the brackets.

Final word, after you've done it enjoy the music ;^)

FWIW, the designers of the Continuum turntable (quite expensive?) and it's integral stand said that despite the massive cost, effort and design simulation that went into creating the stand they admitted it was still a poor substitute for a good wall shelf (!)

I must be a pervert - I still use a table ;^)

The Audio Beat's review of "The Scuttle" rack states that Silent Running decided it was "time to make an affordable rack" so they pitched the basic model at $6600 upwards.

Some vendors have a skewed understanding of the word "affordable"....

Yes, I take your point but the context of this discussion is turntable related. Not many people wish to get down on all fours to load the next LP. ;^)

By way of example, my rack was custom built by the manufacturer (I was the first customer to ask him to make a tripod arrangement) so he drafted it to my dimensions on a CAD system, sent me some dimensioned visualisation files for approval then made the rack for less than 1/10th of the price which Silent Running's cheapest rack is costing.
Good VFM?

Racks are a clear case of "horses-for-courses". Some manufacturers e.g. Grand Prix Audio, will insist on knowing what the various loads on the rack will be so that they can customise it by tuning it.
Praiseworthy though this approach may be it only deals with synergy at a simplistic level (i.e. no real-world AB testing with operational equipment)

Turntable support synergy is so unpredictable that the simplest mechanical features can radically alter the sound. I doubt that there are any guarantees that an esoteric table costing megabucks would even top a group table in a 5 or 6 way face-off against bargain priced opposition.

LOL! (Did Symposium choose the name before ISIS chose theirs? ;^)

There are even worse names in the audio industry though, Schiit for example?
For their sake you always hoped it wasn't... :)