Hi Lew,
With applying for your WHV I would do it online. It is very quick and won't take you too long to get a response (couple weeks max). I believe that the visa will only begin when you enter Australia, you can set the date you would like it to start when you do the application and you can have that to coincide with your landing date.
Coming from England the same as myself, I bought 3k and spent it all in the first couple of months. It is EXPENSIVE here. Especially the backpacker lifestyle. I'm talking 150 quid per week in an average hostel. 5 pound a pint vibe. Everything is more expensive here in Australia (except for chicken breast and petrol lol).
I wouldn't even worry about your farm work yet. I would get the first visa out the way, come over, enjoy yourself, do some travelling, see if you like it and then look at farm work options here. Much easier when you're in the country. You can also set up your bank account when you are here and banks accept your hostel as address - most is done online here anyway.
Cheers