As you stated, theres no need to upgrade the AV-8B Harrier II because there are none, they all have already been upgraded to the + variant, the only airforces that operate the vanilla Harrier II are Italy and Spain, but IMO, gvien the timeline we are talking about (2015-2020), the already modest electronics capabilities of the + variant would be considered archaic (the radar is the single track AGP-65 which are getting very old). In addition the stress on VSTOL fighters would mean that the airframe itself would need to be refurbished. To extend the lifetime of any Harrier II, an engine refit is also most likely in order with new Pegasus 11-61/Mk.107s and within the next half decade or more a newer variat could be developed as well.
The APG-65 is taken straight off the F/A-18A/B Hornet, and many of the early Hornet's in service in Australian and Canadian service has had their radars replaced by the APG-73. Theoretically, the APG-73 could be moved onto the Harrier's, and an avionics modernization program can take place, as the primary targets would be the mission computer and the stores management system. However with the latter, the USMC is upgrading their Harriers with a MIL-STD-1760 bus system which would allow the carriage of more smart weapons.
The airframes would otherwise need refurbishment, but, given the time constraints and costs, it will be cheaper to do such an upgrade compared to developing a brand new VSTOL fighter.
Furthermore inducting the AV-8B into the force proved to be quite a problem for the USMC; it is difficult to fly and has a tendency to crash. Through analyzing their mistakes and training the pilots they were able to correct this but Taiwan might face similar problems.
This has seems to be the case with the USMC, but with the RAF and Fleet Air Arm, crashes are less frequent. The reasons suggested why is that British Harrier pilots have backgrounds flying helicopters before, and that the British purchased their two seater Harriers at the same time as the single seaters, while the USMC only purchased two seaters after their single seaters.
Simply buying more foriegn aircraft will only be a short term solution, especially refurbished aircraft like the A-4SU Super Skyhawk whos service lasted less than 10 years.
What needs to be avoided are radical upgrades of very old aircraft; the A-4SU's are an example of that. For the Singaporeans, getting 31 years of service out of the A-4 is impressive, but the original airframe is well over 30 years of age.
The CF-18 Hornet Incremental Modernization Project is more of what is being looked at, with the key focus being avionics modernization and airframe refurbishment. It is expected that with the upgrades, the CF-18 Hornet will be viable until 2017-2020, when a replacement for the Hornets will be sought.
