If you wanna go for a cheap, good investment, go for the new Sony Clie 360. It might cost a little more, $199, but it's definitely worth it. The Palm m100s were nice, but the clies are better. Specially these new 360s, OS 4.0, Memory Stick slot, rechargable battery, 16 MB RAM. I wouldn't waste money on the m105, cuz you'd end up upgrading soon probably anyways.
It really depends on what you want it for. I wanted a "cheap" PDA and I only wanted it for address, phone number, appointments. I brought a Visor by handspring for $129. I figured by the time I am ready to expand my demands I would then buy a newer more expensive one with the features I then would have come to know. www.handspring.com