In your last schematic (without current limit) you can replace this diode with a small (100Ω) resistor and completely remove the 3kΩ resistor, as in this case transistors can be driven directly from the opamp's output.
There is no problem with 30V output voltage.
The problem is at 0V end.
To achieve output voltage of 0V you will need extra small negative voltage of, say, -5V. This negative voltage (has to be regulated) will be used as supply voltage to opamp (opamp will be then supplied of +35 and -5) and as bottom reference for potentiometer-resitor, so they will be connected between power supply output and -5V.
The other opamp input (+) will be connected to 0V.
Opam type can be LM725, LF411, or any opamp with supply voltage of >40V.
Bear in mind that for 30V output voltage opamps output has to be close to 33V !!!
You already have input and output capacitors, but to make this circuit working you will need to limit the opamps banwidth by adding a small capacitor of, say, 10nF between its (-) input and ouput pins ..
Regards,
IanP