Level difference is a dangerously big factor.
It's been a few years1 since I played Basic and Expert D&D (We never reached Companion/Expert level in our games) but similarly to most other versions of (A)D&D a few levels difference can bring a massive power change. The Magic User and Cleric classes especially have some fearsome spells by character 5th level (Spell 3rd level) that can blow away a host of lower level characters with a single spell2.
This isn't to say that the characters can't defeat NPCs of higher level, but it will be exceedingly dangerous and very very hard. They can level the playing field a little with clever tactics (although tactics and bonuses in B/X D&D are very limited, I highly suggest giving circumstantial bonuses to your players) But certain classes like Magic Users can be exceedingly fragile if caught by surprise and can be defeated, but the magical equipment that these NPCs have (which your PCs will then loot!) will also make a big difference as well, powerful NPCs with exceedingly low ACs may mean that the party Fighters and Dwarves (and other combat types) cannot even hit the NPCs they are facing.
I highly suggest that if you want Important higher level NPCs for the party to face to give them intermediate minions of a level or two below so they can work out secrets of the more powerful ones and have a chance of defeating them by gaining experience from beating up their lower level goons as well as collecting gear to help them get more powerful. It's how I worked when I cut my teeth gaming with B/X D&D and it's served me well ever since.
In a level 1 game Big Bob the Crime Boss is a mighty level 6 fighter, who would most likely easily wipe the floor with a group of level 1 plebs.
However Big Bob has several lieutenants of level 4 and they in turn command several level 2 Bruisers who run different businesses.
With this approach the party can work their way through the lower level goons before facing up to Big Bob with a better than zero chance of being able to hit and hurt him!
TLDR: When the party is level 1, facing solo opponents 2-3 levels above the party's levels is as much as you can risk really without risking TPK3. Characters are exceedingly fragile at these levels. Once the players hit levels 3-5 it gets a bit better but higher level opponents (4+ levels more) especially spellcasters can wipe the floor with lower level groups without even blinking.
1 cough, decades, cough
2 Fireball. d6 damage/level, level 5 is 5d6 damage which even with half damage will annihilate a party of levels 1-2.
3 Total Party Kill