What your describing is called "Buying on Margin". Google that phrase for more specifics. Even though you're not acquiring a 25k loan through a broker, you're still buying on margin. How you acquire the 25k is irrelevant. At the end of the day you're borrowing to invest.

There are a metric shit ton of things that can go wrong with this tactic.

Please keep in mind, I'm not trying to be a jerk here. I'm honestly trying to help.

If you can pay back 25k in 18-24 months (which you stated), why not take some of that money (your money) and DCA (dollar cost average) it into an index fund? Put in a few hundred per month, every month. That way you get your feet wet in investing without going 25k in debt.

Let's say you take out the loan, put all of it into a few index funds, and over the next 2 years we go through a hell of a bear market. Everything drops 30-40%. It's easy to think you can handle that stress, but it's not so easy when you wake up every morning and all of your funds are in the red. Day after day, night after night, you're losing money. Days turn into months very quickly. But you still owe that same 25k plus 3% interest. Everyone thinks they can handle that, until they can't.

That is when people make very bad decisions.

I don't want you to do that.

My advice is to slowly ease into investing. Learn what it feels like to go to bed at night knowing that you lost money, and there's no end to the losses in sight. Sure, the market will come back up. But when?

Don't take out the loan, dude. Start slow. Start small. Make a few bad decisions that cost you very little. You'll learn from it.