If you do order it without a fusion drive, you can always make a fusion drive in the future. If you google "DIY fusion drive" there are tutorials on how to make your own fusion drive using a drive that you installed yourself. All apple does is put a 1TB hard drive and a 128GB SSD in both drive bays and "combines" them using software. There is nothing proprietary about the drives that comprise a fusion drive.
What I was saying is for $250 you can get a 256GB drive and the appropriate cable to add the 2nd drive yourself. This doubles the SSD amount of the fusion drive and would allow more of your stuff to be accessed on the SSD, making the situation better.
Or, if you like having control of where your stuff goes, you can install an SSD and keep the hard drive appearing separately. That way you could choose what goes on the SSD and what goes on the hard drive (for example - you could drag your photo library to the hard drive if the SSD boot drive becomes full).
Finally, if you really see yourself needing a lot of storage in the future, you could buy a 2nd 1TB hard drive for under $100 and install it. In this case, you could set up a RAID0 array. What this does is split up the information to both drives simultaneously, effectively doubling the read/write speed and showing up as a single 2TB volume. The drive that came with my mini was 100 MB/s, so you could see around 200 MB/s in this configuration. As a reference, SSD's are around 500 MB/s. Setting up RAID in OSX is no more difficult than setting up a fusion drive, and there are many tutorials out there. One downside of RAID0 is that if either drive fails you loose your data, so it is especially important to do a backup if you go this route.
As you can see you have a bunch of options after you purchase the computer.