Hi~
I think you can solve the first problem simply. You will use an OP amp, a high-current NPN transistor, and a high-current PNP transistor, so as to make a current limit extended. The OP amp is used as a voltage buffer and to drive the two transistors, i.e., the output pin is connected to the bases of both transistors. The collectors of the NPN and PNP transistors are connected to +Vcc and GND, respectively. The emitters of the two transistors are connected with the negative input pin of the OP amp and used as the output. Of course, the voltage coming out of your computer is applied to the positive input pin of the OP amp. This technique is usually used in the computer-controlled measurement systems.
By the way, which one do you want in the second problem:
V = log I or I = log V?
I think a voltage-current converter will be required, i.e., the input voltage is sensed and and the corresponding amount of current is extracted from the input. If you want a logarithic scale conversion, you may use a diode with the threshold voltage elimated.
Bye~~