The payback is having a perfect chartplotter for myself
Nothing else worked for me.
Practically any relatively modern machine will work. Since we support 3 major platforms - the choice of OS is pretty much open. I use an old Acer laptop onboard, primarily because I won't feel so bad if it gets wet and dies. It is about equal to a modern netbook in it's specs (1.6Ghz previous generation Intel processor, 1 Gb of RAM).
As far as RS232 goes - you need a serial to USB adapter. There are tons out there (literally Google for "usb to serial"), and it tends to be hit and miss. Most adapters are based on Prolific PL2303 chipset, FTDI is the second most popular and more expensive. You may end up buying a few until you find the one that works well. I have 3 or 4 that turned out to work only at one speed, or only for output etc.
Here is a random example: Amazon.com: Cables Unlimted USB 2.0 to Serial DB9 Adapter: Electronics
(search Amazon some more and look for an adapter with good feedback)