Is it ethically questionable for me (an undergraduate) to hire "research assistants"?
All that jumps out at me is that you won't be able to find and supervise someone capable enough for the amount of money you have, which I assume is on the order of tuition or less. You know research is really, really expensive right? People like Ph.D. students - and yourself - do it for cheap out of extreme valuing of their own educational and research experience.

You can't provide a six figure salary (what an industry researcher costs), you can't hire out your own research passions to someone; you have no play.

This, in theory, comes up in the professional world too. As a rule you can't really hire someone better than you to solve your problems... you'll run out of budget for them as they bleed your personal checkbook, or they'll get hired and take over your work (which is probably very bad for you). There's no "ethics", it stops at the pragmatics.

I think the ethics of this are kind of a moot point because there's cultural reasons this can't really come up. A more practical ethical question might be if you can hire researchers to work on parts of your project for you. That's a pretty different question, but I'm pretty sure the answer is yes, that's what a research budget is for (and your undergraduate underlings would of course appear as co-authors, which is a big part of why they took the opportunity).