My advice: installing LE on the internal storage (if even possible on the Wetek Core) can go wrong. If LibreELEC runs fine via the SDcard, simply leave it there. Things won't go much faster when things work from the internal eMMC.
That depends on the size of your media collection. A SDcard of 2GB is an absolute minimum, as Kodi's thumbnail cache will use up the majority of disk space. With an average media collection, 4GB would still be a tight squeeze. And on the other hand, prices of 8GB SDcards aren't that shockingly high anymore.
Just a sidenote;
A pretty good SD-card "SanDisk MicroSDXC Extreme Pro 64 GB" is/was around 12,5€ for quite a while now.
Bought few to have some spares at home...
It is possible to install the WC image to emmc, but I honestly forget how this is done or what the special image filenames are, and WeTek staff are no longer present/active in the forums; so should you attempt it and anything goes wrong please don't ask for help 
Nothing stops you from booting from an SD card and storing media on a NAS drive or USB disk.
