If you work part time, you will defnitely be working weekends. I work 7 days a week on my PhD so would find it hard to balance part time work. However, if it was necessary for cash, or if it was a fantastic job for experience then I'm sure I would manage. You will have to be VERY organised though.
My PhD hasn't really had much structure. My sup threw me in at the deepend in the first year and didn't meet with me for about 6 months. Then I did data collection, had very little meetings and now I am writing up - I am having about 1 meeting every month now. But it all varies, some people have meetings once a week and have a very prescribed timetable of what needs to get done. It also depends on whether your PhD is applied - I've had a load of setbacks trying to get participants for example.