I did holly (have name changed) and I absolutely love it, so convenient and there isn't anything about it I don't like.
I got six gears with reduced ratio, an Ortlieb O Bag (Brompton have the 2015 model bag reduced to £80 instead of £200+ on their webpage) and an Ortlieb backpack and manage to carry everything I need, a laptop and files when necessary and a bit of shopping on the way home.
I wear my Carridice poncho when it is tipping it down (poncho not for the faint hearted) because I had one anyway.
I had a dynamo hub which is fab, I have one on my other bike and didn't want the faff of a non dynamo bike. I just went for the Shimano one (£100 from memory).
I didn't buy the rear rack, I was absolutely set on it (because I have always had a rack) but Cycle Heaven in York advised against it, said the design is such that you are better off adding weight at the front and the rear rack adds a fair amount of weight.
So I just added mudguards, pump, dynamo and bag thing. I have sold my car which is fantastic and rely on this 99% of the time to get to work/about.
I only do five miles most days, some hills, but I don't have the time to walk that far with work and other commitments.
I imagined I would switch to one of my other bikes if I wasn't going to work (nowhere safe to lock it up) but I haven't and my Brompton has become the bike for all my errands during the week.
The place I bought mine from do a hire for a month scheme and then deduct the hire cost off the price of your bike. I didn't do that but I bet other places do the same thing.