Hi,
I am fine to fix all of mentioned bugs. I can fix your mentioned 500 error on categories page, slowness when loading, one page checkout. I have 10 years pro experience on PrestaShop.
Please have a look at my portfolio and feedback from my previous clients before making any hiring decision:
https://www.freelancer.com/u/ranjudhk
==========Why Should Hire Me==========
1) I have proven experience, As proof, I can mention that I have already developed many modules for PrestaShop.
2) All my previous client gave nice feedback and ratings.
3) You will get after-sale support from me for a certain period.
4) You find me on messengers for quick communication when you need
5) I will provide you well documentation of what I do, for you better understand and future track
I am available on the chat box for more details on your project like time, quote, etc.
I am waiting for your quick response.
Kind Regards,
Ranju