If I were to replace my VPI Airies1 with a 4k or so budget?

The variac adjusts the voltage.  There is no need to adjust the frequency.unless there is a problem with the pulley size.  A VPI pulley is stepped to adjust between 33 /13 and 45 RPM.  The 60hz line frequency does not vary.  That's a myth.

 By now, I thought you guys would know that the SOTA  eclipse motor controller system is a highly sophisticated device based on the now discontinued phoenix engineering products. As such it is miles ahead of a VPI SDS and light years ahead of using a variac. But I know this won’t dissuade you. Anyone who is interested ought to do some reading on the subject. My point was that the outboard fly wheel is of some value, however minimal, to regulate speed, and the SOTA eclipse system is at the other end of the spectrum, the best you can get right now in an aftermarket product. I say this as the owner of the original Phoenix engineering system, which has transformed every turntable I have applied it to. I would not dream of having a belt drive turntable without a device of this type, and right now this is the only device of it’s type.

I'm very happy with my SOTA Nova V, but the new advancements in the VI series look pretty amazing, as mentioned, especially the Eclipse / RR package. I'm also a big fan of their vacuum hold-down platters (unfortunately the entry price for a new vacuum table is above your stated budget). If you do move on from your Aries, SOTA would be one to look hard at. Or, incorporate the Eclipse upgrade into your Aries.

In my opinion....

"Speed regulation" is an insignificant factor.  Look up the actual the variation in 60hz grid frequency and what happens if a power generation device is out of synch at all with that 60hz signal.  The 60hz signal is stable regardless of all the misinformation and power mythology floating around.  And there is certainly no audible variation.  By far the biggest pitch variation is by off-center holes in the middle of a spinning vinyl record.  That "wow" is quite audible.  

It's ALL about voltage.  Getting rid of the ac synch pulses that travel through the belt and onto the platter that is critical.  The ONLY possible improvement is an automated means of measuring the absolute minimum voltage that will stabilize a spinning platter RPM.  But in the meantime, if variac trial and error indicate that (for example) 65 volts provides that stability, using an automated voltage regulation that reduces the voltage to 63 volts won''t matter.  

The ONLY advantage to an automated system is that it's automated.  Hit one switch and it's done.  With a variac, the voltage must be quickly manually increased to the starting voltage and then manually reduced to the stable operating voltage when the platter is spinning at full RPM.  Then at the end of play, voltage quickly manually decreased to zero.

In my opinion, any other argument is snake oil.

Thanks everyone for the very interesting posts.  My impression that unless I plan to make a major investment (as suggested mulveling) I should stock to the Aires.  
As for the speed control I'll stick with the SDS and maybe focus on an arm upgrade- and/or a phono upgrade.

So in terms of the latter, the Herron does seem to be a very piece of equipment.  Assuming I were to think about something in $1k range (which would leave me $$$ leftover for the arm) what to you reckon is good and would match well with the Lyra Delos.  My currrent phono is a Trigon Vanguard II which at the team was better most other phono stages up to around $600-$700 with Trigon beinng nonetheless being about $500.  I did a round of comparisons at sound by Singer in Ny wau back when.
I think I should be able to achieve a significant step -up with something around $1k used, no?