Windows will use 20-30GB for a fresh install. You can delete the Windows hibernation file to free up a lot of space too.
With 128GB your looking at 90-100GB of free space with windows 8 installed, and you need to keep 15% of the drive free at least to keep it lasting long as possible, so that brings you down to 70GB of usable space.
You may want to just stick with a mechanical hard drive if your on a tight budget. Windows 8 loads very fast on a mechanical hard drive.