It makes a lot of sense to get a consultant to come in and do a radio survey if you want the best peformance and reliability out of your wireless network system.
But you can do a lot on your own with a handful of simple (and mostly free) tools.
Find that D-Link WiFi Router. Don't plug it into the wired network, just put it in the place you're most likely to put your future central Access Point.
Download and install a utility from MetaGeek called "InSSIder". It's a nice passive WiFi signal strength monitoring tool. It should show you that D-Link transmitting away, and any other devices transmitting in the same WiFi spectrum.
Now take a walk. Go put your laptop on top of the cabinet of each machine you want to monitor, and take some readings with InSSIder. Use screenshots, jot the signal levels down, whatever.
This walkaround process is going to give you a reasonable idea of the signal strength you can expect to see for a typical WiFi radio of consumer-grade output (it's very likely 100 milliwatts, you can check) with a small antenna.
If you find that there are no other WiFi networks running in the plant (you might find some you don't expect !) and that you have a reasonably good signal at each machine location, you can go to Phase 1 and buy and Access Point and some Clients.
Most consumer-grade devices these days are Access Points with Router and Firewall and other nifty things included. They don't perform the simpler task of being WiFi clients.
I am a big fan of Ubiquiti Networks wireless hardware. They're intended for the wireless ISP market and they have some incredibly powerful and durable high-end stuff, as well as inexpensive ones you can purchase online. Cyberguys is a good stocking retailer, but I buy from Metrix here in Seattle.
Ubiquiti's AirOS operating system happily switches between Access Point, Station (what I called a Client, above), Wireless Distribution System (WDS) Repeater, etc. It's really very simple and good.
An example of configuration an AirOS device as a Station (this is what you would do at each machine):
http://www.ubnt.com/wiki/AirOS-Quick_Setup_Guide
I would start with a PicoStation or two, make sure they work, and start building up from there. Maybe use a NanoStation as the central Access Point because of their bigger antennas.
I don't have any good experiences to share with mesh networking gear. Some of that's because I've seen only very inexpensive consumer-grade stuff, some because I was doing WiFi that needed more performance than the mesh hardware could deliver.
Before you start installing anything, you're also going to want to take another trip around to those control systems, this time with the open-source packet sniffer Wireshark installed. Plug into any unused port on the local switch and start sniffing traffic. If you see a lot of Multicast traffic, you know that there are EtherNet/IP I/O or Producer/Consumer transactions going on and that the local switch isn't correctly constraining them. If that happens, don't hook up this machine to the WiFi system or you'll end up flooding it with unnecessary traffic. Come on back and folks will talk you through it.
If it were me, I would do an evaluation phase where you install just the Access Point and one Station in the most remote location, and do some real-world throughput and packet-loss detection.
To recap: INSSIDER utility, D-Link up in the rafters, do a walkaround. Easy, non-technical, and makes you look like some sort of WiFi wizard to the boss, 'cause you're about to ask him for a case of WiFi hardware.
