One of the things we'll cover in session 0 is "Your characters are really somewhere in the 5th-8th level range from prior events, but being locked up and tortured for a month or more has left you weak. It'll take some time to get back into fighting shape." This justifies starting at level 1, and also points out that they will need to level up some simply to get to back to where they "should be."
I am expecting that Irenicus being able to throw around 7th-9th level spells at will (he's really more of a sorcerer than a wizard) will tip them off that they shouldn't go after him at 4th level. The Cowled Wizards using Dimension Door also illustrates an expected power level.... if Irenicus scares the magical police, and the magical police are at least 7th-9th level, the party should be too. The prospect of going into a vampire lair should also make them stop and think "Wait, we need some magical armor and weapons."
There's also the simple greed of visiting some stores and wanting to get a few items off of a shopping list first.
I am generally eyeballing Spellhold as happening somewhere around 9th-12th level and Suldanesselar somewhere between 15th and 19th. ToB is 18 to epic boon territory.... but we'll see what they do.
This is intended as a low-prep game, but I decided I wasn't happy with some of the basic NPC statblocks. Long ago, I ran across a great writeup of what city guards should really carry, including melee and ranged weapons, a billy-club equivalent, a whistle, and some other stuff. I can't find it now. Here's my version.
Spoiler: Athkatla city guard
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Amnish City Guard
Medium Humanoid
Armor Class 17 (Splint)
Hit Points 24
Speed 30 ft.
Str Dex Con Int Wis Cha
13(+1) 12(+1) 12(+1) 10(0) 12(+1) 10(0)
Saves Str +3, Wis +3
Skills Athletics +3, Insight +3, Perception +3
Senses Passive Perception 13
CR ½
Actions
Multiattack. The guard makes two attacks, or attempts to use Manacles.
Halberd. Melee Weapon Attack, +3 to hit, 10’, 1d10+1 slashing damage.
Haft Shove. Melee Weapon Attack, +3 to hit, 10’, 1d4+1 bludgeoning damage. Target must make a DC 11 Strength save or be knocked prone or pushed back 5’ (guard’s choice).
Manacles. Target must be prone, grappled, or restrained. The target must make a DC 11 Dexterity save or have Manacles applied to it. This reduces its speed by 10’, and prevents wielding two-handed weapons, casting spells with somatic components, climbing, or swimming.
Escaping the manacles requires a DC 20 Dexterity check, or DC 20 Strength check to break them. A DC 15 Thieves Tools check is sufficient to pick the lock, but the DC is 25 if the person making the check is manacled. Guards also carry manacle keys in a standard pattern.
Bonus Actions
Whistle. Blows his whistle loudly, summoning reinforcements that will arrive in 1 minute. If other guards are nearby, they arrive in 1d4 rounds instead.
"In the name of the council, I am here to keep order!"
Spoiler: Cowled Enforcer
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Cowled Enforcer
Medium Humanoid
Armor Class 15 (Mage Armor)
Hit Points 62
Speed 30 ft.
Str Dex Con Int Wis Cha
10(0) 12(+1) 12(+1) 16(+3) 10(0) 12(+1)
Saves Int +6, Wis +3; adv. Concentration checks
Skills Arcana +6, Investigation +6, Intimidation +4
Senses Passive Perception 10
CR 10 (PB+4) (level, not CR)
Warcaster. The Cowled Enforcer may cast a spell as an Opportunity Attack, and has advantage on Concentration checks.
Actions
Spellcasting. The Cowled Enforcer casts a spell.
Spell save DC 15, spell attack roll +7.
[][][][] 1st level
[][][] 2nd level
[][][] 3rd level
[][][] 4th level
[][] 5th level
Attack
Cantrip Fire Bolt
Cantrip Acid Splash
Cantrip Ray of Frost
1 Magic Missile
2 Scorching Ray
5 Bigby’s Hand (C)
Control
2 Silence (C)
2 Hold Person
4 Resilient Sphere (C)
4 Banishment (C)
Defense
1 Shield
1 Absorb Elements
2 Blink
2 Mirror Image
Mobility
4 Dimension Door
Summon
5 Conjure Nishruu
Utility
Cantrip Dancing Lights
1 Detect Magic
2 Darkvision
2 Invisibility
2 See Invisibility
3 Counterspell
3 Dispel Magic
5 Scrying (C)
Bonus Actions
Command Nishruu. The Cowled Enforcer commands a summoned Nishruu.
Reactions
1 Absorb Elements
1 Shield
3 Counterspell
"This is your one warning."
Spoiler: Nishruu
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Nishruu
Large Elemental
Armor Class 15
Hit Points 90
Speed 40 ft. (fly)
Str Dex Con Int Wis Cha
14(+2) 20(+5) 16(+3) 6(-2) 10(0) 10(0)
Damage Immunities Non-magic B/P/S, acid, force, lightning, poison
Damage Resistances cold, fire, thunder
Condition immunities exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Saves Int +6, Wis +3; adv. Concentration checks
Skills Arcana +6, Investigation +6, Intimidation +4
Senses Passive Perception 13, Darkvision 60’
Amorphous. The Nishruu moves without provoking opportunity attacks, and can move through openings as small as ½”.
Antimagic aura. Spellcasters inside the Nishruu’s space must make a DC 14 Concentration check to cast a spell. Potions and magic items that must be activated have a 50% chance to fail while in the Nishruu’s space. If they fail, the Nishruu heals 10hp and the item loses a charge / potion is consumed.
Magic Sense The Nishruu senses all magic items and spellcasting individuals within 60’.
Actions
Spell Drain. Any spellcasting creature whose space the Nishruu also occupies loses one random available spell slot (roll a dice of the appropriate size to determine which level). The Nishruu heals for 3 x the spell slot’s level.
Nom nom nom
Enforcers are loosely based on a 10th level wizard, but with NPC HP (d8s) and I didn't check their spells prepared vs what a wizard should have. Really they just ended up with a "find / control" suite plus defenses and some single target damage spells. No big AOE damage spells, as that's what they are there to stop.
Possibly true facts
The Cowled Wizards have several invisible reflectors hanging in the sky that they can “bounce” Dimension Doors off of to reach all of the city. These reflectors also serve as magic detectors.
They have a wing of the main government building to themselves. It’s magically larger on the inside and includes several demiplane sections. Burglary is possible, but very difficult.
The Cowled Wizards have a deal with the Nishruus, who actually live at a secure location on the Material Plane and are kept “well fed” by them, which is why nobody else uses them. There are only about a dozen Nishruus, but the magic that conjures them causes them to vanish just before dying. Death magic can overcome this. If someone starts killing the Nishruus outright, they will get very upset.