Renting lens/lenses for a wedding

A friend of mine asked me if I would photograph his wedding, though I did tell him I do not have the experience or qualification to do this at a professional level of quality so he should curb his expectations, and I agreed because I understand his financial situation.

Now, the camera that I have is a Panasonic GX7 (and I could use my old Panasonic G1 too but not sure if I will find a good use for it, specially the old sensor where anything above ISO 800 is annoying) and only 2 lenses:

Sigma 19mm f 2.8 and Sigma 60mm f 2.8

I have a feeling that the Sigma 19mm f 2.8 will not be helpful in low light situation like the reception so I think I need something better. So I ask what lens do you recommend I should rent for this event ?

Panasonic 15mm f 1.7 (Faster and wider then Sigma)

Olympus 17mm f 1.8 (Faster and widish then Sigma)

Panasonic 20mm f 1.7 (Faster at the same Focal Length and slower to focus)

Panasonic 25mm f 1.7 (Faster at normal Focal Length)

Panasonic 25mm f 1.4 (Very good in low light but not sure about focus speed)

Now I dont know if getting something longer would help since Sigma 60mm f 2.8 should be good enough but still:

Panasonic 42.5mm f 1.7

Olympus 45mm f 1.8

Olympus 75mm f 1.8

I also thought about the premium zooms Panasonic 12-35mm f 2.8 and Olympus 12-40mm f 2.8 but hey are not faster then Sigma 19mm f 2.8 so outside the autofocus speed not much improvement would be gain in low light. But that versatility though

I havent used external flash before (only bouncing the Panasonic GX7 built in one by tilting it up) so not sure if renting one would help me more then a faster lens. Maybe the zooms and a flash would be a better combination. What would you recommend (besides getting a better photographer for his wedding).