Help me know what next purchase should be for Menoth

Have you looked at themes at all yet? The game more or less runs on them now and you will be better off picking a theme to build in then buying models that can be taken in that theme.

draycos is on the nose, you can find theme forces on this page: warmachineuniversity.com/mw/index.php/Protectorate_of_Menoth

75 points is the standard, 50 or even 35 are perfectly reasonable starting points though. Theme forces are crucial because you get free “cards” (solos and CA’s, usually) for every x theme qualifying points you spend. This often means that playing in theme grants you about an additional 15 points. They also give two bonuses, for example Faithful Masses grants you a free wall. Of course, if both players play without themes the game should be balanced, but if one does then it will be an uphill battle.

Our most competitive themes are Exemplar Interdiction and Faithful Masses, followed by Creator’s Might (which doesn’t have many top-tier builds but is playable) and Guardians of the Temple (our weakest theme, but definitely not “unplayably bad”, though it doesn’t see much build variety)

While not the most competitive caster or theme, you seem to have a good start to a Reznik 1 list with Scourge of Heresy, two Repenters and Redeemers, and a Revenger clocking in at 36 points. You could also run a similar list (though with not Scourge due to theme restrictions and character jacks) with Kreoss 1. I don’t know the caster very well, but the Reckoners, Templar, Vigilant, Sanctifier and Castigator may be good with Durst (who can also play a jack heavy game in Faithful Masses)

If you wish to invest in Creator’s might, top of the purchase list I would recommend 1 (eventually 2) units of choir and Severius 0. Severius 0 is amazing and can also be played in Exemplar Interdiction. He will need an attached jack, most people run a Dervish because it is cheap and fast, but a Reckoner has synergy with Road to War and aiming. I would also recommend an Exemplar Warder (can also be played in Exemplar Interdiction) and the Covenant of Menoth (can also be played in Faithful Masses), they are strong scoring pieces with very useful utility. After that, a Hierophant and Wracks are evergreen across almost all the themes. Finally, though an expensive investment, a Judicator is our strongest ranged piece and sees play with a variety of themes and casters

There’s an ongoing thread discussing Reznik 1 builds here: lormahordes.freeforums.net/thread/8399/tactica-high-executioner-servath-reznik

Based on the models that you do own already, I think you are good to field a "Creators might" Jacklist at 50 points (or more I haven't done the math yet). Just something to keep in mind.

No matter what lists you go with you will want a unit of Choir.

Edit:
Creator's Might
35 points (34 total taken)

Malekus
    Castigator
    Reckoner x2
    Repenter x2
Vassel of Menoth
Choir of Menoth (min unit)

Add in a wrack if you have one (or proxy it). Malekus' feat will boost damage on all of the ranged attacks from the jacks. He also has a decent amount of Focus for running jacks. (he can also cast open fire to get additional attacks out of the Reckoners or Ignite on the castigator for a good round of punching.

35 points is an excellent place to start especially if you are teaching kids. (lists with duplicates of the same thing are good too.)

Do you have a second faction already to play against?

Based on what you have, you can easily field a Creator's Might list. At some point you're going to want to expand your 'caster selection, since what you have is a bit limited, but that can happen later. When you do, I highly recommend Harbinger, High Reclaimer, Kreoss3, Severius 1 and/or 2, Reznik1 and/or 2, and Thyra. Any of those will complement your collection and broaden your options. Probably should start with Reznik1, as you have his character 'jack Scourge of Heresy.

A Choir is vital for Protectorate. Get one as your first purchase. I would also heavily consider a Hierophant. He's a great 'caster attachment. You might also consider Severius0, the solo, as he fits in Creator's Might and Exemplar Interdiction, and is seriously good. He runs a mean Repenter.

Since you're also interested in Faithful Masses and Exemplar Interdiction, here are a few good purchases for each.

Exemplar Interdiction: A CA for your Cinerators; a second squad of Cinerators (pick 'em up cheap on eBay) with CA; a squad of Vengers; a squad of Bastions; a squad of Errants + CA (I like them); Exemplar Warder(s); Exemplar Warder Elias Gade; High Exemplar Gravus. In order to get to a good playable state, I'd say at a minimum you need a couple of units (two Cinerators is great, especially with Durst) and probably Gravus for Desperate Pace. Everything else is just gravy.

Faithful Masses: Idrians; Paladin solos (Champions and Dartan Vilmon); Initiates of the Wall. Much as I wish it were otherwise, Zealots don't perform how you want them to as they have a short ranged threat range, poor accuracy, and tend to die in droves before doing much of anything. If you want to make Zealots perform, there are a handful of 'casters who can help you out (High Reclaimer, Severius1, Feora3) but usually you'll get more bang for your buck with Idrians instead. Plus, you already have Deliverers, who perform that role better.

To complement your Warjack selection, the Crusader kit is a good buy. Crusaders are a fine budget 'jack. An Indictor wouldn't steer you wrong either. These are only fun purchases, as you already have a workable stable. Our lights are great too. Consider the Dervish, especially if you pick up Severius0. I also personally love the Redeemer. He gets work done every game for me.

You can also look at several recent events for Protectorate stuff to get a lay of the land of what people are playing. I'm not advocating netlisting as a general strategy, but it is useful when you're trying to figure out what to purchase with limited funds.

As mentioned, you need to get choir. How you got that much Protectorate without Choir is rather miraculous.

You could use some different Warcasters too. I'd look at maybe Vindictus and Harbinger. Along with that, you should get some Knights Exemplar and their UA(2 units if you can) and some of our Exemplar solos. The Warder and Exemplar Seneschal. Then you can make a nice Exemplar focused list with Vindictus.