Good qualities of ovens/microwaves for baking cake
I'm not going to claim this is an exhaustive list, especially as I'm not a commercial baker, but a few things that I would personally consider:

What is she baking?  What size vessels does it need?  How tall does it rise? This will affect how can she fit in the oven at one time (how many per shelf, how many shelves for the height it needs).  Note that you don't want to pack them too closely, as you need space around them, even for convection.

What is the largest sheet pan it can fit with a couple of inches (5+ cm) around each side?

If electric, how much power does she have available?  (will it require getting an electrician in to run heavier wire?)

I'd also look to see if there are any places that deal in used restaurant equipment.  In the US, restaurants have a rather high failure rate, and there are places that specialize in liquidating restaurants / auctioning off everything in them.