Does it have inbuilt USB? If it does not, 10.3 will simply not install, saying it is unsupported. If this is the case, then I would reconmend getting a PowerMac Blue & White. I bought one last week (Stock spec G3 300 with 17in Monitor, for $399), and one today (G3 350Mhz, 20GB HDD, 512MB RAM, 10.3 for $350 delivered). These are awesome machine that can be had on the cheap, and run Panther fine.
If this is not the case, and it has inbuilt USB, then provided you have enough RAM backup (1GB would be ideal for a slow machine), then it will run fine. I run Panther on my 350Mhz iMac, and it runs quite fast, comparable to 9.2.2 on my 233 PowerBook. Provided you have the RAM backing, and it has inbuilt USB, you are set for 10.3.