If you're sitting near the front any tape recorder with a plug-in microphone and adjustable input levels should do it.
The automatic level adjustment thing is called Automatic Gain Control and is a bit of a mixed blessing, as sudden loud noises (eg. someone coughing near the mike) will cause a temporary drop in recording level of the signal you actually want.
You're never going to get a brilliant recording of a lecture unless the lecturer is wearing the microphone him/herself, as the sound source will be at some distance from the mike and there will always be competing background noise.
The quality of the mike is crucial, as is the ability to adjust the input level of the recording device to make up for the fact that you're some distance from the speaker. I haven't encountered special lecture recorders, but there's no magical way of overcoming distance from the speaker and variable signal levels. Have a look at the upper end of the dictaphone market if you don't want to spend hundreds.