1st level human wizard (PC)
3x guard NPC
1x acolyte NPC
This is the player's first time, and it's a solo campaign. The first mini-adventure involves recovering the cargo of a supply caravan that didn't make it to her magic academy due to bandit hijacking (as a sort of graduate-test). The guards and healer are on contract from the school to assist her. The acolyte is essentially just to hang back and tend to wounds and probably won't engage in combat unless someone goes down or the PC is seriously wounded.
So, with that "party" makeup, how do I determine the XP budget for a "medium" or "hard" encounter since there's no clear CR->level mapping in 5e for those NPCs (who will certainly factor into any combat as meatshields and melee damage)? I was thinking about reverse-engineering them as crumby lvl2 fighters (given the 2 HD worth of HP they have), but they certainly aren't on par with a lvl2 fighter, given the lack of action surge, second wind, etc., so the encounters would end up being too hard compared to PCs or DMPCs.
Could they be lvl1-equivalents, essentially trading any abilities a PC would have for extra HP? How would this work at later levels with higher CR NPCs (veteran, priest, mage, etc.)? I'd rather not have to build a bunch of DMPCs as hirelings, because I want the NPCs to be a bit vanilla compared to the versatility of my player. I'm already introducing 1-2 DMPCs to the game for her to take control over as she gets a feel for the flow of the game (for now, she'll handle them out of combat, and I'll effectively show her how to utilize them until she's ready).
Just to be clear, I'm comfortable with the monster-generation rules for crafting NPCs as enemies to determine their CR, but I'm confused on how to use them in the adventuring party when budgeting encounter difficulty XP.