convert pwm to analog signal with a load of 0.7A
Disregarding any potential heat development, can I simply add a mosfet or a transistor such as the TIP120 darlington behind the opamp without screwing up the voltage level on the tracks? Or will this affect the voltage level in a negative way (which I suspect)

Yes. Actually, op-amps are really good at this! You can put any kind of amplifier you want circuit between the op-amp output and the feedback point (where the - input connects) and the op-amp will adjust its output to keep the + and - inputs the same. That means it automatically compensates for any voltage changes caused by the amplifier!
What kind of amplifier should you use? For your circuit, an NPN pass transistor should do. Collector -&gt; power supply, emitter -&gt; tracks (via current sense), base -&gt; op-amp output. This is called an &quot;emitter follower&quot; configuration. Note that the emitter will be about 0.7V lower voltage than the base. The op-amp will compensate by setting the base 0.7V higher than the voltage you ask for. But the op-amp can't set the base higher than the op-amp's own power supply, so this does affect the maximum power. You'll have no problem putting 12V on the tracks if you bump the power supply up to 15V or so.
You could put an NPN transistor in the negative track instead of the positive one - then your minimum drop will be about 0.2V instead of 0.7V - unless the negative track has to be 0V for some reason. You could also use a PNP transistor in the positive track, in which case you also have to swap the + and - because lower base voltage would make the track voltage higher. Either of these make a common collector amplifier, and if you try them, then do put a resistor in series with the base.
By the way, your current sense circuit drops voltage too. Connect the + input after the current sense circuit. That way, it'll compensate for the voltage drop caused by the current sense circuit. And again, the op-amp has to have a high enough voltage for the compensation to be possible.

Is it a good idea to pick a stronger opamp such as this one from Mouser which can drive up to 1A

Sure. If it does what you want, why not? I think you'll learn a little bit more by making a more complicated circuit with a pass transistor, but it's your choice.

Is it a good idea to use this circuit (with 5V instead of 12V) to 'replace' the potentiometer of an existing buckconverter (essentially I'd be controlling the output of a buck converter using this circuit as input)

That depends, how is the potentiometer wired in the circuit?