Firstly, the game can become somewhat easier precisely *because* of the ability to hire high-level NPCs (and, to a lesser extent, the Dragon) at a relatively early stage. If you don't do this, you eliminate most criticisms about the game.
Having said this, the challenge in MM games has always been the early and middle games - the Dragon makes this relatively easier, but soon enough you get to the point where the Dragon's attack isn't the easy path to finish encounters that it was, and there's still enough content after this point to make the game enjoyable IMO. In addition, unless you're going to re-read and plan from the start, it will probably take you a while to find the Dragon anyway, reducing this factor: the Dragon-is-OP argument really applies for subsequent playthroughs when you know exactly where to go to pick him up asap. If you just stumble on him normally, I wouldn't say he's at all game-breaking.
Finally (for point 1!), high level characters are already levelled up and have used skill points in a pre-set way, and almost certainly NOT the way a standard min-max player would have spent them. In most cases, by the time you are able to hire a higher-level character on a class that you'd want to play, you'll have already developed an earlier character more efficiently anyway.
Secondly, I'd start with your single character, as intended, rather than a full party (at least for your first playthrough). The early game is designed for the characters available at that point, and editing a full party would make that part easy.
Finally, the characters available for hire early on are a Knight, a Necro, and a Cleric (also a Vampire, I think, but I never use them. If I'm not going to stick with them through the whole game, I'd rather not split the XP with someone I'll be getting rid of eventually). Your main trap disarmer in the game is the Dark Elf, so they are a good choice for a first time playthrough to balance out your early party, because there are some relatively high-level traps in the early game and you can concentrate on Disarm, while the other characters cover the other main roles. Additionally, the most well-known early available NPC is a Dark Elf, so if you're going to avoid picking her up (again, assuming like the Dragon you've pre-read and have decided where to go in the first place), the Dark Elf is a good choice to stick through for the entire game