The history of the deck seems odd, since I don't think the Systemdek IV existed in 1980. The Systemdek III that it replaced was developed from the original late-70s Systemdek and relied on springs in compression for the suspension. The lower cost Systemdek II was a development of that in a 'skeleton' plinth, and I bought one of those in 1983. The manual covered both decks.
The IV followed on from the IIX (springs operating in tension) and related models, and the IIX came in around 1984 or so. I uploaded a couple of brochures some time ago, showing both the early and later ranges.
None of this really matters, as these are excellent decks in any case. Whichever
top-of-the range Systemdek you have, it's still well worth servicing and bringing back into use
. Furthermore, apart from the electronic power supply, which sometimes gave trouble, there is little that generally goes wrong. New belts are readily available and are the main item that needs replacement. I ran my Systemdek II for about 20 years, and it's still in use with someone else.
The AT1120 is a low-mass arm better suited to higher compliance moving-magnet cartridges than moving-coils. In practice, most of the MM cartridges currently made are aimed at medium-mass arms such as the ubiquitous Rega design, which has about double the effective mass of the AT1120. However, given that the deck has spring suspension, the lower mass of the arm should aid stability and resistance to footfall.
Depending on budget, you have many choices among MM cartridges. An Audio Technica model like the AT440MLa should work well, though some find it a bit bright. The long-established Goldring 1042 is perfectly happy in Rega and Linn medium-mass arms, but is not desperately fussy, based on my experience of it. It's quite expensive these days, however. The low mass of the arm also means that slightly heavier cartridges, like the Ortofon 2M models or some of the Nagaokas, could potentially be accommodated, though they are also rather bulky and might not fit in the headshell.