Not that sure about the Challenger 2 for a lot of reasons. First thing, would it have existed at all? In the first place the Chally 1 was to be used
only by Iran, and was taken over by the British Army after the revolution. Had they been accepted by the first customer, what would have been the main tank of the British forces?
Also, it sounds like the early Challenger 1 was plagued by teething problems. How would these have turned out,were the tanks in service half a world away in a not-so-important country? Would the after-market service have been as good as for tanks in Britain? Considering all that, would Iranian tankers keep a fond recollection of British tanks? Good enough to purchase the next one? Upgrading the Challys once purchased goes without saying, but surplus Leo2s (just a thought) would be available soon enough for cheap and give NATO compatibility and upgrade potential.
Thinking about it, I'd say look at the Turkish army and defence industry right now, and apply nearly the same to Iran. Up to the revolution they still had lots of less-than-excellent equipment still running (large army, mind you) which would bear interesting upgrades. Even the AIFV would be a good candidate for the next APC/IFV.
Apparently the JCSS file confirms ZSUs in Iran from the 70s onwards. Given the JCSS also cites SA-6s from the same period, I'd say this is a nice commercial fallout of the 1973 Arab-Israeli war.
As you said, there wasn't much of a Western offer on systems of this level back then anyway. There isn't much still, but in the 80s, how about a nice batch of Chieftain-Marksman SPs?
Other than that, there are a couple of European SAM series that could be of interest, like the Roland, Crotale or Aspide.
There were also a number of Rapier SAMs in service, which could have been upgrade, including to SP Rapiers. I've once seen a pic of a purported domestic SP-Rapier fix on parade, dunno how useable and widespread it could be. SA-15s might be part of the answer...
Thinking about F-15s, there would be a large number of F-4s from the 60s to replace as well, so why not?