Expressing/pumping advice please!!

So my 4 month old is normally EBF but I have some medical issues with liver and need to have an MRI scan on Monday that requires IV contrast. I was told I need to dump my milk for the first feed after the scan so need to have some expressed milk from beforehand. I also do want to start building an emergency stash in case this happens again.

Do you have any manual pump recommendations? I don’t really want to invest a lot of money into an electric pump at this stage.

I’m also confused as to when I should be pumping or hand expressing the milk. I did buy a Haakaa to catch any excess from the other boob but can only get 30mls. Do I breastfeed and use a manual pump on the other side? Or do it in between feeds? I’m so confused!! Or is it more like I pump when I bottle feed her? Haha I’m so clueless! Help please!