YMMV, and a unique new kit is always welcome, but from Wargames Atlantic's current inventory, it's hard to beat any/all of the following as a starting place for gritty, low-level "old school renaissance" adventurers (or their NPC equivalents):


(Late Roman Legionaries for warriors, fighters, merchant guards, mercenaries, paladins, etc.; lightly armored in chain shirts and helms, with basic swords, shields and spears.)

(Foot Sergeants for relatively low-rent warriors, fighters, mercenaries, guards, militia, etc. The armor is relatively light with chain shirts and sturdy caps, and this is the set to look for for crossbowmen, backed with a fair selection of different melee weapons.)

(Peasant Levy are great for rogues, thieves, rangers, bandits, cultists.... Peasant clothing and simple weapons like bows, daggers, clubs, and improvised weapons.)

(Goth Warriors for miscellaneous guards, villagers, militia, henchmen, "Level 0" heroes, etc.; with just cloth armor and simple weapons and caps.)

(Foot Knights make great, well-rounded, and well-funded fighters, warriors, knights, cavaliers, blackguards, paladins, etc. in full chainmail and tabards. This set probably has one of the better selections of generic fantasy melee weapons like maces, hammers, swords, battleaxes, etc, and is surely THE go-to set for full helms.)

(Classic Fantasy Guards of course make good guards, fighters, warriors, paladins, etc. in plate mail; there are bits for female characters in this set.)

(The Conquistadors have some rather flamboyant costumes under sturdy breastplates and helms, and are a great source of halberds, rapiers, two-handed swords, heavy crossbows, and even early guns. These guys would make an interesting starting place for bards, fighters, royal guards....)

(Classic Fantasy Villagers make great villagers, of course, but also great plucky adventurers with humble origins, bards, rogues, spellcasters, and more. Includes bits for unarmored female characters.)

(Classic Fantasy Villagers II supplies some elf heads that will likely work with most of the other kits here for elf characters, and also dwarf bits, and more bits for bards, spellcasters, rogues, and other eccentric characters. Includes bits for unarmored female characters.)

(One of WGA's first kits and still one of the best available for halfling fighters, warriors, rogues, rangers, clerics, and "burglars". There's a complimentary digital-only set of Halfling villagers, too, that might be of interest for halfling and gnome characters.)

(Dark Age Irish Warriors are great for barbarians, druids, etc.)

(The Afghan Warriors adds a nice "Arabian Nights" flavor for adventurer rogues, monks, spellcasters, rangers, villagers, bandits, cultists, assassins, etc., with scimitars, bucklers, turbans, daggers....)

(Much like the Afghans, the Sepoy Mutineers adds an "exotic" flavor of turbans, fezes, sabres, daggers, bucklers, and the like for rogues, bandits, cultists, assassins, villagers, and the like.)

(The Boxers are another nice choice for an exotic "Big Trouble in Little China"/"Kung-Fu Theatre" flavor on your villagers, assassins, etc., and this is surely a fine go-to set for fantasy monks.)

(Warring States Armored Infantry make for some unusual but fine warriors, mercenaries, guards, etc. in scale mail)

(And the Persian Infantry also make some unusual but ferfectly good warriors, guards, mercenaries, etc. in padded armor.)

(The Damned Female Infantry - certainly not an obvious option for traditional sword-and-sorcery fantasy by default, but this is a fine source of female bodies in breastplates, jackets,trousers, and sturdy boots, with some solid female head options, and even a couple melee weapon arms; you can use arms from any other kit to finish that foundation up as female warriors, fighters, paladins, clerics, rogues, and the like.)

(The Baron's War Retinue is a somewhat pricey investment, but supplies sprues from most of the Baron's War sets already mentioned, including the Knights, Sergeants, and Peasant's Levy, as well as a command sprue that adds bits for building a crowned king/nobleman, mitered bishop, standard-bearer, and trumpeter, and even includes the bits for a handful of mounted knights. A great way to cover a number of rank-and-file warrior types at once!)
(Militant Monks would make superb old-school clerics, druids, and wizards, as well as cultists, in robes (with or without hoods) and armed with staves and daggers - you'll want to source maces for old-school clerics from elsewhere, or craft them with a little creativity, and some Sorcerer's Spoils will supply bits for wizards and other arcane spellcasters.)
That's a LOT of great fantasy and fantasy-adjacent sets, and the Digital Atlantic option provides even more options for those of us with 3D printers, while I suspect that most players are probably better off turning to e.g. Reaper, 3D-printed STLs, or something like HeroForge to choose or create dedicated adventurer models. (These boxed hard-plastic sets shine, I thnk, when they're used for putting dozens of rank-and-file mooks like monsters, guards, mercenaries, bandits, cultists, henchmen, etc. on the table, with leftover bits for creating inventive villains, but maybe aren't the best option for building adventurers with.)
Still, between WGA's Militant Monks and the Baron's War Retinue alone, I'd say you'd have a lot of ground covered for a virtual lifetime supply of classic fantasy adventurers - just add Sorcerer's Spoils for spellcasters' spellbooks, potion bottles, scrolls, and other paraphenalia!
From there, the halfling militia and the two fantasy villager kits add "meta-human" characters, and hard plastic kits from competitor brands like Frostgrave (cultists, soldiers, knights, and wizards), Stargrave (for the alien heads), and Oathmark (I really like the light infantry dwarves, elves, humans, and goblins) add even more variety. Everything else is listed above is just gravy.