You an find a lot for <$700 if you shop correctly. Watch for coupons and possibly buy them on ebay for a few bucks at the most.
You can watch for dealzmodos on Gizmodo.com,
Try these too:
http://slickdeals.net/http://www.techdeals.net/Unfortunately, you will have a problem with RS232. Most laptops don't include them anymore. You'll need to buy an USB to RS232 converter. I suggest you buy the Kensington one, I've found it to be the most reliable and have the most drivers, including Macs. Should be less than $40.
Honestly, for your relatively 'meager' requirements, you may get away with a netbook. You can get a Dell Mini 10 for about $350. IIRC, the 10v supposedly has HD graphics and HDMI out. You can run Windows 7 on this type of machine and it'll scream. I have a Mini 9 with OSX and Win 7. Dell Mini 9 & Win 7 blows away my Thinkpad T42 running XP.
