Libra is still in it's development stage. You can't buy Libra now.
And most importantly, you can't invest in Libra. It's a stablecoin. Its price would be fixed and the fluctuations would be minimum.
i.e. you'd gain nothing by holding it. So, you can just buy it when you need it and sell it when you don't.
Right, it is still in development stage. Wait, when they will announce their announcement and publish their whitepaper, roadmap, then you can think for investment.
Libra is planned to be launched in early 2020. However, it is not yet known whether Libra will be present in all countries completely or not