Apple lets you download 7.0.1 or 7.5.3 freely and legally ...
The distinction between bought Reference Releases of Apple's System Software (upgrades) and free updates has been rehearsed here many times, most recently in the last week.
Apple did (to its credit) make the update 7.5.3 into a standalone installation that had no requirement for a prior installation of 7.5, thereby effectively making it into a quasi-Reference Release which is freely downloadable. In the course of that it muddied its own distinction between RRs and updates, but it has been a boon to many people who otherwise were without a System 7 installation for old Macs (and especially PPC Macs).
7.0.1 arrived to support newer PBs and Quadras, and in that and a few bug fixes it was and is an update, despite that it was available on five 1.4MB floppies and accompanied by an 800kB tune-up floppy (1.1.1).
A Plus with 4MB can notionally support 7.5.5, which is easily reached with free downloads of 7.5.3 and 7.5.5, but I did not suggest that to pvolkmann because there is much code in there on which 68000 processors can and do choke. If the OP wishes to do so, alternatively, there are massaged/pruned versions of system Software from 7.1 to 7.5.5 that will grace a Plus rather than garrotte it. If a drop-in and go System is needed, however, I should stick with 6.0.8, as I have done with a 512Ke/2MB that has access to an HD20 and SCSI.
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