If you do one express, at the same time each day, you'll have plenty in reserve (prob too much, so maybe 2-3times a week). Then if you don't want to express much at work, you've got something to fall back on.
Maybe just check you milk freezes well before building up a stash. My LO is only 6 months, so my expressing amounts, will not be relevant for you.
By 9 months your supply will be fine, so if your LO is feeding more than once in the day and you only express once/none during your work day 3 days a week, it shouldn't matter. If its not enough, you could either run down your stock or carry on expressing in the evenings over the week to build enough for those 3 days.
While a lot of 9 months do only have morning and evening, I think it's a borderline age and you can't assume this.
Anyway I'm rambling, I think its hard to second guess now what your LO will be doing 2 months down the line.
As for the room, get it and then just use it to rest for 20mins
. It also helps to set a precedent and hopefully help other mothers to be down the road