If I was to take an Intstrument Rating to my SE Piston PPL licence. How bad weather can I be prepared to be able to fly in?
Apart from the legal limits on SE IFR:
a. minimum RVR for SE IFR operations is 800m and
b. approach minima
you will be hampered by icing as no piston singles are certified for known icing.
Beyond that it really depends on what you want and feel comfortable with.
Single engine at night/imc/over water/inhospitable terrain.
If you are happy with that, nobody will stop you. Of course for the aircraft it does not matter whether you are in IMC or flying on a gin clear VMC day.
FD