I spoke to the company and they said it didn't need a new drier as it didn't burn out, it seized due to lack of oil because the gas leaked out. Seizing presumably doesn't contaminate the system.
I also removed the cover from the outdoor unit and it already has an EK-053S drier already fitted, which is soldered in and doesn't seem to have replaceable anything. According to the net, it is a loose bead type, and is about $20-40 USD (or $49 in AUD). It doesn't look like it is a large capacity drier to layman me (see pic - it's not very big, and there are no ports to measure the pressure drop either).
http://www.flickr.com/photos/totoblu...7600545952409/
The suction accumulator is a bit rusty too - don't know if that is a problem but it could be a potential leak.
The compressor is a YHE6-0500-TFDC Copeland. Seems to be about $600-800 USD on the net.
I asked about replacing the whole thing and they said about $7-8000 for a similar sized Mitsubishi ducted inverter system, which would save about 25-30% on electricity.
They did come up with another idea which is to replace just the outdoor unit. York can supply replacement outdoor units that are compatible with old systems like mine. Provided the price is reasonable, that sounds like a good idea for resale. Might get a new thermostat fitted at the same time and it'll look like a whole new air-con system :) (and it will be, almost).