OS 10.6 uses very little space. I don't remember how much I had when I bought my MBA but I have MS Office, 60GB worth of music, and all the regular suspects, and still have 35GB remaining. MS Office is relatively small.
There are things you can do to maximize your SSD. These will only save a few GB's here and there, but it's worth it. I only installed the MS Office applications I needed. I don't use Outlook or Messenger or Powerpoint, so I only installed Excel and Word. I use IMAP with Google mail so that all my mail resides on the server rather than on my computer. Sure, you sacrifice the ability to work offline, but I've found that there are so few places where you're actually off-line, and when I am (like on an airplane) I actually relish the quiet.
There are programs you can download that will help you get rid of unwanted language files. If you use a lot of pictures or video, I'd suggest getting a compact USB drive (Like LaCie's Riki, which is really little).
If you're realistic about what you'll be putting on this thing, 128GB is more than enough space. Take a hard look at what programs you're using. With WiFi all over the place, I realized I didn't need to horde programs. If I only used a program once or twice a year, it's easy to just store the license and then install the program on an as-need basis. I figure that by the time I outgrow the 128 GB the prices on OWC's 256 replacement SSD will be down.
I don't know if there are any savings available if you buy it preloaded but I would imagine that if you did, they would have to include restore disks for any software you purchased preloaded. A restore disk (or USB key in this case) should return your computer the the exact state it's in when you turn it on, and that should include any preloaded software like MS Office. But I've never bought it that way and don't know for certain.
Good luck.