I haven't tried playing in exactly the fashion the OP is suggesting so no real tips to give, but I'd say it'll mostly be more meticulously time-consuming and If that's what you're going for, you should get it in spades. I've always largely ignored merchants (even for ammo) so no problems there. In terms of
combat, however, most of that won't make much difference (no HUD is the only thing that will a bit, imo). No matter what (vanilla) settings you run with, if you want a bit more combat challenge (vs. roleplay-reality challenge, 2 different things), my main tip would be to self-gimp skills/weapons/perks.
Sometimes I play with zero HUD but more often I have it set to only 1 'bar' in the Settings. 1 bar makes it almost useless 90% of the time when running frantically around in combat, but if I squint really hard for a few seconds, still allows me to read item text (most of the time...). Not being able to read item text doesn't sound that bad, but over time it can be very annoying. To be honest tho, the original/main reason I don't like HUD isn't for difficulty, it's because I can't stand looking at it...it really pulls me out of world-immersion seeing those giant HUD thingies at the bottom of the screen. Loathe it, and I don't need red dots to tell me where enemies will be. It's also one reason I turn dialogue text off.
I don't use crafting for ammo, since I don't care much about max bullet dmg - so can't speak to how difficult it is to keep yourself supplied in that fashion. But I also don't buy much from merchants, since I don't care about the 'big ammo guns' or non-regular ammo much at all, and the rest is easy to stay stocked via looting/finding.
While at this point I've tried/played HC during some solo games, I find while it does make certain aspects of the game more of a meticulous chore, it doesn't make
combat harder, and that's what I personally desire -not roleplaying reality. Turning off the HUD & not being able to instantly keep track of your health meter is a bigger combat-handicap than "RealityCore". So I don't actually use it much - and never when I use companions.