Using a mixer incorporates ingredients better and results in fluffier, more uniform dough. It's also a must for mixing smooth icings and glazes. You'll need the paddle attachment and the dough hook. This is what I have: KitchenAid KSM150PSER Artisan Series 5-Quart Mixer, Empire Red.
You'll also need a big pot for frying. I use a Lodge Dutch Oven. The enamel-covered dutch ovens come in pretty colors, but they're alot more expensive. The Lodge Logic L8DOL3 Pre-Seasoned Dutch Oven with Dual Handles, 5-Quart is around $30. I originally bought it to make No Knead Bread from the New York Times. If you haven't made No Knead Bread yet, I highly recommend it! It's easy, delicious, and another great use of your dutch oven. I also think it would be fun to take it car camping, throw it in the fire and cook something amazing outdoors. There are entire cookbooks devoted to doing this. Maybe that will be my next endeavor.
I don't have one of those awesome electric fryers that regulates the temperature of the oil, so I have to use a thermometer. But that's cool, because I need the thermometer for checking the temperature of water and milk before adding yeast for raised donuts. Plus, donuts are finicky about oil temperature. If the oil is too cold, the donuts soak it up and get soggy, but if the oil is too hot, the outside becomes too crisp before the inside cooks! Ugh! So that makes your thermometer an important tool. I opted for the probe thermometer for no other reason besides the fact that Alton Brown said it was better. That guy is the ultimate food nerd and in my opinion, food + nerd = sexy.
Metal tongs for flipping donuts while frying. You can find these anywhere. I have OXO Good Grips 12-Inch Stainless-Steel Locking Tongs. It has a knob at the end to lock them closed which is a must for frustration-free storing.
I have an awesome metal spatula that has a super flat edge that is handy for transferring donuts from the cutting board to the oil without distorting the shape. If you click on the picture for details, you can zoom up and see that there's almost a knife blade edge on the end so it slips under dough very easily. It works way better than any plastic spatula I've ever had and I use it for almost all of my cooking. It is metal, so you obviously can't use it on a non-stick pan, but we got rid of all of those a few years ago and now we just use seasoned cast iron skillets. I was so surprised and impressed with the non-stick-ness of cast iron pans. Anyway, back to the metal spatula. It wasn't cheap, but it is my staple tool in the kitchen, so I've come to terms with the $25 that I paid. The one in the picture is the Victorinox 3-Inch by 6-Inch Chef's Slotted Fish Turner, Walnut Handle, and I think it's funny that they call it a "fish turner." It just seems a little off-putting to imagine handling donuts with a spatula adorned with charred fish chunks from last night's dinner. At my house, we just call it "the flipper."
Rolling pin. You have one already. If not, get one of these ones without the handles so dough won't get caught in places that are impossible to clean out.
Get a Ateco 5357 11 Piece Plain Round Cutter Set for cutting round donuts, and a pizza cutter for cutting raised bars and a 2-gallon bucket for texas donuts!
Baked donuts can be sticky. I used a cheap donut pan for years until I realized that maybe I shouldn't have to wash, dry, re-grease and re-flour the pan in between every six donuts (some of my recipes make two and a half dozen so I was doing that five times). I splurged an extra few dollars and ordered this silicon-coated USA Pans 6-Well Donut Pan and it was totally worth it.
For transporting donuts, I wanted a bin with a handle on the top so I could carry it one-handed and I liked that I could snap on and off levels depending on how many donuts I planned to transport. The Snapware 6032 Large 2 Layer-Cupcake Keeper has a removable cupcake tray and has more than enough clearance for donuts.