If you've been in the industry long enough for a facility to call you in when the designers have fled, you probably don't need my advice about how to document your work, communicate extensively, and prevent yourself from taking the blame for the mistakes of others.
So I'll stick to the technical things I would do if I were in your shoes.
Obviously you'll need RSNetworx for DeviceNet. The software took a big leap forward about the same time as Studio 5000 v21, so don't assume any old version will work well on a workstation with modern Studio 5000 and RSLinx. Get v21 at the very least, and ideally the current release of version 28.01.
You'll be able to do most of your work just by bridging through ControlLogix backplanes or via the USB port on a 1756-DNB. But you might want the ability to do updates, downloads, or configurations of devices that are removed from their running networks, so you should look into getting a standalone network interface. The 1784-U2DN is the modern USB device, but a good old 1770-KFD with a decent USB/RS232 interface will work as well. Get a decent 24V bench power supply that you can use to dedicate to DeviceNets, rather than powering the PLC and the I/O with it too.
If you think you're going to be doing very low-level compatibility testing or protocol analysis, get the -U2DN because it's supported by Frontline Test Equipment's NetDecoder.
Like it or not, you're going to have to deal with Electronic Data Sheet files. Make a directory on your project repository and make sure you put copies of any third-party device in the facility's EDS there. Heck, also put any A-B EDS files that you end up having to download or get from Tech Support and weren't in your install of RSLinx Classic.
If you can get your hands on a Molex NetMeter, it's going to be useful principally for determining if the network itself really is the problem. Yes, it's handy for wiggle-testing connections to see if the error counters jump, but in my experience I mostly used it to prove that the network wasn't "failing" or getting "noisy" when devices were malfunctioning, allowing me and the other engineers to properly discover when things were instead being powered own, disconnected, or submerged.
Get some terminating resistors, 120 ohm, 1%. Get some that fit whatever round or flat quick-disconnect media you're using too.
If you use PanelView Plus terminals, take some time to look at the Sample Code website for the DeviceNet diagnostic faceplates. They're not the most aesthetically pleasing GUI ever made, but they're a lot better than asking somebody to stand there looking at the scrolling indicators on the scanner module.
