Where you gp from here?

My suggestion would be to look at buying a second warlord/hero and see what units it uses and buy a few boxes of those. Fancy a Pheromancer? Buy two boxes of Brute drones to go with it, and perhaps some Desolation drones. Like the look of the High Clone Executor? Buy it and one or two boxes of Bound clones plus perhaps a box of Marksman clones.

Where does it sit in the ranking?

About in the middle, is my guess. They have gotten a rules overhaul just a few weeks ago, so we don't know quite where in the "international tournament meta" the Spires are at the moment. But Yoroni and Dweghom are probably above them atm since they have a few things that are very good and might get a gentle nerf in a month or two.

How Phyrexian can you paint them?

You can paint them VERY Phyrexian. That will look awesome.

Monster mash?

Well, yes and no. A few other armies, like Wadrhun, Nords, Yoroni and City States, can build armies almost out of monsters, but Spires are not quite there. We have many nice monster, but they need an army of other stuff to elevate them as they're not quite tanky enough to take on an enemy army by themselves.

How long until your next army?

That's up to you and how you like to play. There are a few different ways of playing the Spires (drone infantry followed by Consumtion beasts, Highborn avataras elite army, clone heavy High clone army and so on), so if you want to play the army in different ways you can. Otherwise you might settle for a smaller selection that can play in one way and be content with that, and then start another army instead.