First and foremost, you need a good liability insurance policy.
After that, if you don't have your supplies already, you need at least two sets of clippers (roughly $200 to $500 each, depending on brand), lots and lots of blades in a variety if lengths (roughly $30 per blade), supplies for caring for said clippers and blades, good quality shears ($100+ per pair), good quality brushes and combs, a good grooming table (around $200 for a super basic model to $3000+ for an electric lift table), forced air dryers (around $200 for a basic model to upwards of $1000 for a high end model), crates or cages for dogs to stay in when they aren't being groomed, and a raised tub (either built in or free-standing) with an easily cleaned hair trap.
For storage, you can mount wall cabinets or use freestanding storage cupboards.
