What important tools does a small tailoring outfit need?
It sounds like she already has a basic &quot;starter kit&quot; for sewing. So, instead of getting her a comprehensive kit, I would try to focus on one or two high-quality items that would save her time, or make her work easier in some way. Here are a few suggestions.

An adjustable-size dress form. Make sure you get an adjustable one, because presumably her clients are not all the same size. The link I provided is for Singer, which is generally a good brand of sewing supplies although I haven't used their their dress forms. Here's a blog where they reviewed different brands.
A serger or overlocker - this is a machine that looks like a sewing machine, but instead of sewing a seam it trims and wraps the raw edge of fabric. It's a huge time saver compared to putting in a hem.
A set of multiple rotary fabric cutters in different sizes. This is the tool that looks like a pizza cutter that your wife already has, but she might appreciate having more than one. Or a new blade for the one she already has; they do get dull over time, and you don't even notice how dull your current blade is until you use a sharp one.
A cordless iron.
A set of thread in all the colors, so she doesn't have to buy new matching thread every time she starts a new project. Make sure it's good quality thread that flows smoothly and doesn't stick and get caught and make a giant snarl every 5 minutes.
An abundant supply of sewing machine needles. So many that she can switch needles whenever the old one gets dull without feeling at all guilty.
A better table, chair, or lighting for her workspace. This works best if it solves a problem that your wife has mentioned to you. If you want to install something in a way that will make dust, that probably shouldn't be something you surprise her with, but if you do make sure the sewing machine is back inside its case and fully covered. Ditto for any fabric storage cabinets.
More storage space. There is not, and never has been, enough space in the home of any sewer for all their fabric. I'm not just talking about buying her a new cabinet; I'm talking about clearing some junk out of somewhere in your home and saying, this space is now yours. And maybe put in a new cabinet or some shelves. This is the least expensive suggestion, but the most work for you. As an added bonus, when your wife sees you working on this, tell her you're clearing out the space for one of your hobbies (preferably one she finds annoying). She'll be extra pleased when she finds out it's actually for her.
A gift certificate for a tune-up for her sewing machine. It looks like Sears will do repairs on a Kenmore, but I couldn't tell if they also offer adjustment and cleaning. They have DIY manuals on their site, so you could study up and do the tune-up yourself. Be sure to use the right kind of lubricant and not just whatever you have lying around (tip: 3-in-1 oil is not good for sewing machines).

Whatever you get, make sure it's high quality. Do some research and find a brand that has a good reputation for both quality and customer service. Make sure they have a good replacement policy in case the item breaks at some point in the future. If you buy from someplace like Amazon or Ebay, check whether the manufacturer will still honor the replacement policy (ask me about the broken set of knitting needles for which I can't get a free replacement because the gifter bought them on Amazon instead of directly from the supplier).
Make sure she actually has space to use the item and space to store it. That might mean you need to get into her workspace and take some measurements.