We used stock tall white cabinets from Lowes, which come in various configurations. They don't have a lot of choices, but you can buy them with a few different configurations to meet your needs.

I like the idea that they have doors on them instead of open shelving, so it keeps some of the dirt out. We leveled them screwed them to wall, and added lower baseboard trim that made them pull together like one long cabinet.

Bruce

1) For your budget of $300-$500 max, use the metal shelving you already have with the addition of new $20 flip-top plastic storage boxes. Closed boxes will keep the dust out. The flip tops can be easily vacuumed to keep the laundry area much cleaner than with open boxes and milk crates. Measure your shelves, then pick the right size to fit two or three per shelf at the store. https://www.amazon.com/Akro-Mils-12-Gallon-Plastic-Storage-Attached/dp/B00006IARN

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2) Add a $50-$100 simple upper cabinet with doors for spray bottles, laundry detergent and anything else used more than once a week.

3) One of your milk crates can make good storage in a car trunk. The others can be donated to avoid the temptation to reuse in the garage- they are hard to clean when they get dusty or dirty.

4) A $75 shop vac or a $30 stick vacuum can make garage laundry area cleanup easy and quick. It makes a big difference in how often the garage floor gets cleaned when you don't have to lug the house vacuum out to the garage.