Um, it's really really hard to know how you are going to feel after your gorgeous bundle arrives.
Even your work has to legally provide somewhere to express, realistically they aren't going to, unless they want you at all costs to work for them!
Part time work and expressing is totally possible, I've been doing it since Ruby was 6 weeks old, but she is my second child and everything is very different. I have found expressing at work less than ideal, and most people I know seem to give up breastfeeding at 6 months if they are working full time. I do express in the toilet as it is all that is available, but it's more taking the time out to do it that can be frustrating as I want to finish my work so I can get home on time, not sit in a toilet expressing milk (btw it's fortunately a very nice toilet!!)
Lastly, working part time is very very different to full time. You have to communicate a LOT more, and depending on your job, you may feel like you do a half a$$ job at work and half a$$ at home which can be very depressing! Remember babies don't really fit into a timetable or scheme, They don't all feed 4 hourly on the hour for example, and sleep through the night and etc etc.

Personally, take the part time option, if you change your mind I'm sure it will be available full time, but it's much harder to go in the opposite direction. If you want to breastfeed, I think having 3 months at home where you are not worried when and where you breastfeed is a fantastic start. My daughter is looked after 2 days a week and has a bottle (formula) during the time she is cared for, unless I have a spare expressed 180mls. The other days I breastfeed as much as possible so my milk isn't affected by the 2 days reduced feeds.

What am I trying to say? Plan away, it's not a bad thing, but just be really really flexible with yourself. I think the 3 months at home, and then part time is a good plan. Start there, but don't commit yourself to anything else till you see how you go. And remember, jobs will come and go (they really do!) but your baby's life only rolls through once!