ADS provides you with nearly everything you need for application development. The only thing you'll need to purchase is an ATA flash card. This will allow you to transfer files and load new operating system images onto your development system. You may think you don't need it, but trust us, you will need one of these at some point.
You can buy ATA cards from many sources. Get one that's at least 16MB. We recommend 32MB for most development needs. Compact flash with PCMCIA adapters are very common now. Buy.com has a good selection. We use Viking and SanDisk cards regularly at ADS.
If you don't have an ATA card reader, there are many on the market. We generally recommend USB card readers, as they require less cabling, transfer data quickly and don't tie up the parallel port.
Note: If you have an NT development system (which doesn't support USB or internal readers) or Win98 (which isn't very reliable with USB), look for a reader that connects to your parallel port. One that's proven to be reliable is the CameraMate.