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[Marcello]

As I see the alternative would be:
1) Ordering a Challenger 2 lookalike, regardless of whether the british Army is on it or not;
2)Buying new Leopard 2;
3)Buying something else like Leclerc or Abrams or whatever.

Option two might be the best but would the germans sell to the Shah?

[PlasmaKrab]

There are two things that may make more tanks necessary past the Challenger era: 1000 tanks may be overambitious as an order, leading to numbers being cut down at some point, and 1000 are probably not going to be enough for the whole army.
Bear in mind that to this day, the IRIA is reported to number something between 350 000 and 500 000 active personal (Pasdaran included), as opposed to probably nearly 1 million in the 70s (350 000 left with 65% desertion rate reported in 1979, grain of salt not included). Even with these figures, reports give between 1600 and 1700 tanks in current service.

I don't know how many tank units were in active service before the revolution, but with such a force, I'd say 1000 tanks is only enough for maybe the best half of the armored units. Like in many countries, rear-echelon units, mech divisions and the like would go on running with older material for some time. And it's this material that would stand replacement or upgrade in the 90s-2000s.

I agree that the British Army would certainly go for the Challenger as well at some point, but I wouldn't call Iran buying a full batch of Challenger-2s later on 100% certain.
Anyways original Challengers with regular updates would be enough to hold the high ground against any neighbor to this day and for the foreseeable future. So buying a new full batch of Leo2A5/M1A2/Challenger2/Leclerc generation in the 90s may not be top priority. We should probably look or a later replacement like what the Turks look like going for these days (i.e. brand new 4th-generation tank).

Point remains about what to do for the rest of the tank fleet. That's where cheap surplus Leo2s and M-60 upgrade packs come in play. There should be at least 500 60s tanks to replace, upgrade and/or convert. Of these quantities (maybe there were significantly more of them before the war?), about 1/3 is Chieftains, 1/3 M-47/48 and 1/3 M-60s, all pre-revolution.
IDK how many of those would remain in service after the introduction of the Challenger, but there would certainly be some pending upgrade

[Marcello]

The problem with buying Leo2A4 is that while it may be good in the short term you are going to end up with a block obsolescence issue in the long run to deal with, as both would have roughly the same age and tech level.

"That's where cheap surplus Leo2s and M-60 upgrade packs come in play. There should be at least 500 60s tanks to replace, upgrade and/or convert."

For modernization purposes the focus would rather be on the 900-1000 (or whatever, sources seems to differ ) or so Chieftain tanks in service, rather than the M60. I would guess some upgrade, perhaps locally made, to bring them to standars comparable to those of late marks of Chieftain. Then some thought could be given to the M60. M48/47 would be definitively junked. I would say 1000 Challenger, which would still account for a reduction from the original order which was for 1200 Shir 2 IIRC, the 900 or so upgraded Chieftain, say a couple of hundreds of
Chally 2 or whatever and some M60. That is nothing to laugh at. The total armed forces strenght was 500.000 by 1977 IIRC but I cannot cross check ATM.

"brand new 4th-generation tank"

Do you mean T-95/140mm gunned western tanks?

[PlasmaKrab]

You're right, there must have been either high attrition during the war or a typo in one of my sources. 1000 Chieftains would be the bulk of the tank force to replace after the inception of the Shir. Upgrading them to Mk11/12 standard or above would be interesting.
Of course, this considered, the whole Patton fleet would be a secondary concern.

What I meant with a 4th generation tank was a brand new post-2000s one. Only working example I can think of is the Korean K2. The Black Eagle is another project but not yet in its final stage. Make the base requirement 120L55 gun or better, high mobility, hunter-killer/autotracker FCS suite with 2nd-generation TI on both sights, active protection... on a newly designed basis. Don't ask for more details, I'm not too good at this generations game. The idea is just to get a newer design than upgraded 70s/80s tanks (M1A2, Leo2A6, Challenger2E...) which would be not much better than whichever upgrade you could lay over the Shir 2.

[Marcello]

"You're right, there must have been either high attrition during the war"

Attrition of Chieftain was very high indeed, the iraqis ended up with whole lots filled with them.

"Make the base requirement 120L55 gun or better, high mobility, hunter-killer/autotracker FCS suite with 2nd-generation TI on both sights, active protection... on a newly designed basis"

Frankly nothing of the above requires a new chassis. The gun can be fit in a Leopard 2 or Chally 2 with no problem. The rest is electronic stuff you could fit on pretty much anything.

[Marcello]

A new chassis is really needed only if you want radically better protection and a design optimized to take a 140mm from the start.

EDIT
Well you could stick a 140mm in a Leo if you wanted (it was done) but I suppose it might be better to have an optimized design with an autoloader from the start. Uparmoring the tank to survive 140/152mm hits is an other matter. Therefore you will probably need a new chassis with some exotic configuration, otherwise there is no need. The K2 is basically an equivalent of the
Leo 2/Chally 2 with a few funny features.