Only $200K ?!?
That's what?, 2-3 programmers' salaries for only one year? And at the end of that year, you will have a full-featured, well-documented, bug-free product that runs on Win95, '98, NT, Win2K, XP, ME(?), under any service pack, and on any hardware? That's so user-friendly that you won't need tech support (or can justify having the customer pay it's full fare)
We do that now on our motion controller setup software and ,as Steve Luft company does, we give it away for free. I can justify this because it is also a marketing/sales tool, diagnostic tool, manual, etc. The PLC package would be grafted on to current setup tool. You can get it off the internet if you want to see. The cost is obviously borne by the person that buys the hardware. I learned long ago that it more acceptable to sell software/firmare in hardware units that people can touch and see.
What's the communication link? RS-232? Will a USB or PCMCIA card be able to handle it, too?
Ethernet or RS232. That is already worked out. USB is not industrial strength and has limited cable length. Ethernet is the future. Customers love Ethernet and just about every laptop has Ethernet now whereas serial ports are going away.
I think you'll be hard-presed to pull it off (depending on the size of that 61131 subset).