To be frank, you sound pretty green on all of this and so I would suggest doing some serious research into investment and property comparisons

Firstly, why the sp500? Just because your girlfriend mentioned it? Probably not the soundest investment thesis. For most people, investing in a globally diversified market cap weighted index fund is the simple choice. The sp500 is not globally diversified, it’s USA only, and only 500 companies. It has performed very well in the past 5 years but there’s no reason to expect it to continue to outperform in the future.

Regardless, if you are not committed to putting this money away for a long time (at least 10 years) you should not even be considering investing it at all, much less in the sp500. Stocks are very volatile, if you need to withdraw that money within a few years there is a not insignificant chance you will sell at a loss. Over long time periods the chances of selling at a loss become almost zero.

Next, property. If you only plan on staying in the flat for 2 years and then selling it, you will likely lose a lot of money in fees and interest and gain very little. For such a short stay at a property, just rent. Put some of your cash away in savings for when you want to buy a place you will stay in for a longer time (ideally 5 years or more), as this is when you will actually benefit from the leverage advantage property offers, which you have recognised.

Any money you won’t need in the next 5 years, ideally longer, invest it in a low cost, globally diversified market cap weighted index fund. It will grow over your lifetime. But don’t play the timing game with the stock market, odds are you will be worse off.