You won't need to delete whats on there to add to the iPod.. I've done that dozens of times. In fact, the BIG problem was that, using Apple's software (or the third party bundled stuff) you could not copy the mp3's back off the iPod.. It was a one-way transfer. I fould a neat freeware program (for the Mac) called iPod Browser 1.5 which will allow backups, I'm sure something similar exists for PC's. As to the price vs. space issue, when I got my 20gb iPod, the big difference was that the 20 included the dock, iPod case, and a few more extras. Once you figure out the price on them, it was worth it.
As for Apple's track record on batteries, their new laptops are the BEST on battery usage I've ever seen on a machine. Life cycle (even when using wireless network, DVD drive, etc) is great. I've repaired PC's for about 8 years. Worked on PC's since MS DOS 3.x, and was a confirmed Mac hater. Job market sucked, and an opening appeared on a Mac network in a school system. Now through the job I've got a nice Gateway laptop with all the extras, and a more basic iBook G4. The G4 is what comes home with me, it does a better job of seamlessly and quickly hopping networks. Arfcomm's new look doesn't quite work on the Safari browser however.
Oh yea, if you're going to run the iPod on a PC, you should get yourself a cheap firewire card at the same time as the iPod, and realize that if you keep the thing in the dock forever and a day, every time you cycle the power on the machine it's a life cycle on the battery.
As a previous poster mentioned, there is a software update for them that does address battery issues, but IIRC it's more extending the time between charges than extending the overall battery life. They did the same for the iBooks, and it made a huge difference. Apply the update before you get to transferring music. Should the battery die down the line, you can replace it yourself with available instructions and a $20-40 part, or pay Apple $99 and they will replace it or send you a new iPod (usually the new iPod unless you've had it etched on the back). Screw the applecare policy.