Well the RAN could certainly get a fleet of useful frigates from the Type 21 build you suggest, my suspicion though is that they wouldn't be as cheap as you imagine, particularly if they have Seawolf or Sea Sparrow.
Whether they'd have the same problems as the RN
Amazons or not would depend on how the RAN managed the program. In the UK there was a politically motivated drive to keep the RCNC (Royal Corps of Naval Constructors) out of the Type 21 design process completely to "save money", but that also made it hard to impose tough standards and practices on the builders because those standard came as much from the "culture" of the RCNC as they did from anything formal that could be written into a contract.
Problems with the Type 21s:
1. Hull cracking.
The combination of an aluminium superstructure and a steel hull made it tricky to analyse hull fatigue, but it was believed that the hull was safe even if the superstructure cracked right through, which was good because it started cracking really early and had to have fibreglass patches applied. Then just before the Falklands, new analysis showed up a previously unrecognised failure mode that could snap the hull itself, and all the ships were warned to "avoid excessive sea motions" just before setting off the fight a war in the South Atlantic.....
The
Amazons did more than their fair share of the NGS work in the Falklands because they had the Mk.8 gun and the latest Seacat system but were more "expendable" than the Type 42s that were needed to defend the fleet with Sea Dart.
Arrow ended up with a damaged hull due to both the heat she endured when she came alongside
Sheffield after the Exocet attack and to the sheer amount of gun firing she did and the stress that put on her hull. She ended up having to be sent back to the UK before the fighting was over, and all the surviving ships had to have large steel patches applied to the sides of the hull in the centre section after the conflict.
2. Damage control/resilience.
Experience in the Falklands showed that the RN's institutional memory about damage control had slipped a bit generally, but the Amazons had some particularly bad features and unfortunately, the fatal attacks on
Ardent and
Antelope gave plenty of opportunity to explore them. Some of these problems stemmed directly from the commercial contruction techniques that had made then so attractively cheap in the first place. Cables wern't glanded where they passed through bulkheads but were instead fed through a wide, common slot, which allowed smoke to spread. The ship's ventillation system was very simple and couldn't isolate one area of the ship from another, so smoke in one area rapidly spread throughout the vessel. The ships only had one major databus with no backup: in the first attack on
Ardent, a bomb went right through this databus and severed it, leaving the ship to face the second attack with only her two 20mm Oerlikons.
3. Inability to upgrade.
Adding new weapons and sensors to the ships proved impossible because anything that was added would make the ship sink lower in the water, and freeboard to the quarterdeck was marginal. Put a bit more weight on them, and it would have gone underwater in a hard turn.
Most of these problems got through because of the political interference in the normal design process which made the ships "cheap". To avoid them, the RAN would have to ride the builders' backs far more closely and impose higher standards on the design process, but that would doubtless result in large slices of the cheapness going away.
If you must have a Vosper Frigate, then I'd definately make it a larger one, similar to the Mk.10
Niterois bought by Brazil. After all, if Brazil could afford to buy/build seven hulls in the 1970s (4 x ASW, 2 x GP, 1 x training) then presumably Australia could have afforded a decent number of them. I'd go for the CODOG powerplant too: the extra weight low down helps with stability and the Mk.10s had much better range than the
Amazons.
By the way, an
Amazon wouldn't have a smaller crew than a
Perry:
Type 21 : 177
Mk.10 : 217
FFG-7 : 176