Even if you never intend to get Horizons, it's worth holding onto at least a few of each type of data and material. There has been at least one CG recently that requested some types of data and materials, so there are likely to be more at later dates.
And if you do later get Horizons, you will already have a head start.
But you will quickly find that 1000 materials and 500 data don't take very long to fill.