It is your second guess. en refers to a quantity, a rough translation could be some.

The coma is still used quite often, but as it disappeared orally it influenced the writing and people tend to skip it. Though it sounds weird, it could be roughly translated as:

We got some, luck!

as in

Luck, we sure got some!

It is almost not used in english tho, and you would more easily say:

We sure got lucky!

I hope I can help you. (TLDR below)

In fact, in this case : « On en a de la chance. », the « en » acts as a pronom personnel. It is a quite particular one like the « y » you can meet in « J’y suis allé ».

Both are used as a replacement for a complement (or even fo aslice of a sentence that could be huge)

« Je suis à cet endroit, je reste à cet endroit. » –> « J’y suis, j’y reste. »

« J’ai de la chance. » –> « J’en ai. »

The en and y are used when everyone inside the conversation understands at what these two pronouns refers to. They are totally implicit.

In your particular case, one could argue that it is a grammar/syntax mistake because they repeat the same information twice (en is supposed to replace de la chance ) In common french, people used to do it to put more emphasis on it.

TLDR : « en » is a way of implicit replacement of a part of the sentence everyone inside the conversation understands. Two letters that could replace from few words to a whole paragraph. In your example, the en and the words it is supposed to replace (de la chance) are both present. It is a mistake often made in common french to put emphasis on something which is so said twice.

The comma could be used if you want to be a proper grammar nazi. 🙂

« J’en ai, de la chance ». It becomes now a perfectly ponctuated sentence but semantically, it is still a mistake.

The form : « J’en ai de la chance » is still accepted and is certainly going to be the norm.