Think you are right to have lighting challenges on your mind for an indoor event. What lenses are you using these days? I seem to recall you looking at putting some money into glass a while ago.

What is the lighting like inside the garden? Will time of day be a factor or is it all lit artificially?

Reason I ask is that if you don't have portable lighting and the light there is insufficient, though being a garden I'd expect better than average, you will likely need to commit money to wide aperture glass or a portable studio to overcome the issues if you wish to pursue the opportunity and not blow your local reputation on your first job. Customer expectations will dictate which way to go. Do they want studio look or candids?

Have a feeling Sean and some of the others will be better able to guide you to the particulars in terms of equipment selections as they seem more in touch with what is available.