Duane, with that budget, I think you're going to be limited to looking on EBay for the machine you need, especially if you intend to run SmaartLive or audio editing or otherwise resouce-hungry apps. While we all want to stick to a particular budget, sometimes you have to pony up what the market is selling the necessary hardware for, even if it means holding off on your purchase until you have the necessary funds, as I've done more than once and with literally clenched teeth until I can buy what I need.
Having said that, I've had good luck buying a couple notebooks from EBay and usually from an individual with a good rating.
Definitely avoid Vista like the plague. Unless something drastic happens, I foresee a cottage industry for XP in the years to come, much like the industry that sells used NEVE recording consoles (Yeah, that's probably a stretch, but really, you can never tell). You're not liklely to find a recent machine with serial ports, but I would look for a PC card-to-serial port adapter instead of the usual USB-to-serial adapters and you can get multiple ports from one adapter if you need it (socketcom.com is one source). The USB-to-serial units have been a crapshoot for me, though there are good ones there. And what's good for one application may not be good for another. Believe it or not, I've had good luck with the RatShack unit.
The good thing about buying a used machine is that you can get up and running, then buy more RAM and bigger hard drives as you need it.
Lately, even though I'm a ThinkPad/Levono fan, I've found some really good deals on Gateway's refurbished site. Last April, I bought a $1400 convertible notebook/tablet for $750.00 and I swear, there wasn't a mark on it. It didn't come with a lot of software, but I usually have no use for what comes on a new notebook anyway. And it's been a great machine. I'm haunting that site for another one to replace my contrary Toshiba as we speak.
Hope this is of some help..
Geri O