Please note that I did say that the difference will be "modest" if you put your data files onto an SSD. Some will argue that keeping your data on an SSD is a waste of the SSD's space, and you'd be better off sticking with a HDD.
I could quite easily have managed with a 128GB SSD, as I have a couple of "backup" HDDs in my system. However, for the price premium (and we're not talking about a massive cost here), it's nice to have enough SSD capacity so that you can not only install your OS onto it, but you can keep your data and apps on it also.
You have about 60gb of dropbox data and maybe a few other bits and pieces. Your Windows install will be taking up about 25gb, and with basic apps you could well be using around 30-35 gb.
One possible solution (the one I would choose anyway !) would be to go for a 240 - 256gb SSD. You'll be able to install Windows (or clone your current SSD) onto it and have plenty of room for all of your data. Your existing SSD could be used as extra storage, or be sold or used in another system/laptop. I think that if you go for 128gb, you will be not leaving yourself with much spare capacity. Yes, you could keep your existing SSD in your system, but the extra ~£50 that it will cost you for ~250gb over 128gb is worth spending, IMHO. I think you'll see a slight improvement in overall performance due to having a newer SSD and having everything stored on it. You'll also benefit from not having to be concerned about running out of space anytime soon. If your budget is restricted, a 128gb SSD would not be a bad idea, but I just feel that if you are going to "upgrade", then a larger SSD is a better bet.