barbaraaa: actually I'm not sure you need to replace your wardrobe before you can start doing this. We often think we need way more than we do to look smart / casual / different than usual. You will do fine with two pairs of really good trousers to start with, maybe a white blouse and and a good pair of pumps or whatever suits your job. Try to pick the absolutely most urgent pieces for now and go on working from there. I think the most important thing that you try to set yourself goals and try to stick with them. Of course there might be a moment when you absolutely want something that you didn't plan. That's fine. But let these be exceptions and be "under control" most of the time.
Thank you for replying Since July I have already bought two pairs of black trousers, a black blazer and a few blouses, and I've recently ordered a black skirt. I've been really careful not to buy anything more "casual", as I already own a lot of that, and I also bought things I actually needed. If I get the job I'm aiming at I will have to buy actual suits, however.
That being said, I'm not much of a shopper, except for during the sales: but for the past three seasons I have been more careful with what colours, styles, etc I do wear and suit me, rather than going "oooh pretty! cheap!". So I really think this could be a thing for me, just I might have to go beyond the 4/5 pieces/season limit