The Problem
Assuming that the guards in and of themselves are not worthy of being Paragon-tier, what magic items would best justify giving them Paragon-tier stats, narratively? I want to convey the sense of "unearned power" in their hands. I know I could just make them be Paragon-tier, but the narrative really suggests that they haven't trained, or acquired the experience, or been blessed by a deity or demon or anything– they just have good gear.
Of course, I also don't want the players to suddenly jump to Epic tier power levels when they inevitably loot the corpses of these jerk-face guards. And artifacts are probably a bad idea, since I want the gear for these guards to be standardized, and there are a lot of guards.
So what should the items be? I'm also willing to convert gear from earlier or later editions of D&D if there's something else out there that would be appropriate.
Fluff Background Information
The guards worked for a defacto Magocracy with a nominal civil government, until the mage's guild was destroyed via secret plot (an epic level Lich transmuted all the wizards into ingredients for some evil plot related spell). This left the nominal civil government rudderless, and the figurehead decided that the best thing to do would be to literally run the city based on bribes. To make this bad idea worse, the richest members of the city outside of the wizards were a number of rival criminal gangs, who now compete with bribes for control of the powerful city guards.
The wizards, before their deaths, were renowned throughout the lands as being the finest makers of enchanted weapons and armor around. The guards themselves were not trained or enchanted themselves, as the wizards used their elite guards as advertising for their skills as enchanters.
Recently, in addition to hassling the civilian population of the city, aiding various gangs in their shakedown activities, and press ganging poor tradesmen and farmers into going monster hunting so they don't have to, the guards have been hunting a couple Vampire Lords (servants of the Lich) in the city. These Vampire Lords have given the PCs a serious run for their money over the course of multiple combat encounters, and are known to be heavy hitters.
The Vampire Lords are very cautious around the guards, so it's pretty clear to the party that the guards really are heavy hitters too, if their existing reputation wasn't enough.
Crunch Background Information
The five PCs are level 16 right now, range from very well optimized to merely moderately optimized, and my group enjoys running "Very Difficult" combat encounters as standard.
Their classes are Cleric/Shaman1, a Wizard2, a melee wildshape-focused Druid, a Barbarian, and a Swordmage.
1: Optimized character #1, a healer more effective at healing than I have ever seen at any table of D&D, ever.
2: Optimized character #2, a very effective tactician, with powers that complement his tactics quite well.
Best Answer
On a comment, you stated:
Yet, we need a way to make this work from a story point of view without creating an opportunity for a huge power boost for your players. So, why not change things up just a little bit?
The Iron Man suit doesn't make anyone stronger. It uses its own Strength Score.
Doesn't matter if Tony Stark, Hawkeye or Pepper Pots is inside the Iron Man suit. It still has Strength 30, because it is the armor that is doing the work, not the person inside. We can use something similar to this concept.
The research was mostly correct, but it failed on one point - their magical gear isn't actually standard magic gear at all. Instead, the gear is a construct of sorts that shapes itself in the form of armor and weapons, to be equipped and used by a creature. The creature gets unbelievable power, but pays a price that isn't yet apparent to the group - maybe a shortened lifespan, maybe a brainwashed servitude to a faction, maybe the inability to leave a certain area... pick your poison. The truth of the thing is - who is actually fighting isn't the guards, when they don their gear. It is those special constructs that their wear. The guards themselves work just as pilots and power sources.
Mechanically, the gear those guards are using isn't buffing them. It is replacing their stats. While using their gear, the guards have a paragon-like stat block, with all the abilities that entails. Take the gear away, and they revert to their heroic power level. It is the gear-construct that fights, not the person inside.
If a player decides to don the gear, what happens is that their character sheet gets replaced by the one the guards were using. They lose access to their own individual powers, their own scores, and instead are given access to the powers of this standardized, mass-produced gear-construct until they take it off. It might be useful, and might be a power boost for a little time, but once they start leveling, this gear will be eclipsed by their own abilities, making it obsolete.
Add some sentience to this magical gear-construct, and you can create even more opportunities for roleplay and plot twists.