I am just starting to dabble in this stuff, so can't really help you, but have you thought about the BASIC Stamp?
You can get the Stamp plus a board, serial port connector, cable, and programming environment/debugger/complier from Radio Shack for about $80. There's various Stamps, including one designed for unattended datalogging (more EEPROM, lower power consumption). Plus there is a very good set of forums at
http://forums.parallax.com/forums/default.aspx
I'm planning on using my Stamp to enhance an old camera (motor drive, light meter, etc) but haven't had time to really get going on it - plus my only laptop with a serial port keeps crashing.