There's a lot of post under Cinematography here suggesting good lenses for the BMPCC4K. For beginners (and even some of the pros here including myself owns one of these lenses at one point), instead of buying the speed booster, use that money and add a little more and buy one of these MFT lenses.
Olympus 12-40mm f/2.8 PRO Lens - https://www.bhphotovideo.com/c/product/ ... al_ed.html
Panasonic Lumix Vario 12-35mm f/2,8 Power OIS - https://www.bhphotovideo.com/c/product/ ... vario.html
and if you have a little extra left, this Panasonic 25mm f/1.7 is a cheap lens and nice for filming in low light - https://www.bhphotovideo.com/c/product/ ... f_1_7.html
That should be your starter set.
If you insist and want to go the speed booster route, then for starters, get a Metabones MFT to EF .71X speed booster and a Sigma ART 18-35mm f/1.8 EF lens.