1) On what date can I actually send my ILR application?
The earliest you can apply is 28 days before the date you entered the UK in 2012.
So, if you entered on 13th Sept 2012, you can apply for ILR on 16th August 2014 and you have until the expiry date of your visa to apply.
2) Now that I've passed my 'Life in the UK' test, is there anything I need to do leading up to applying for ILR?
You will need to provide 6 letters addressed to both of you, evenly spaced, covering the entire 2 years of your spousal visa.
So, you want one document in each name, for each 4-month period between Sept 2012 and Sept 2014. They can be jointly addressed or separately addressed, as long as you have each name on 6 documents (so, 6 joint or 12 separate, or a mixture of both).
There is a list of acceptable documents on the SET(M) application form for ILR.
You will also need to provide your most recent 6 months of payslips and bank statements to show your income.
Otherwise, all the documents you need to send are listed on the SET(M) form.
3) Do I have to get a biometric residence card before applying for ILR or do I schedule something once they've received my application?
You won't get the residence card until the ILR is issued, because it IS your ILR visa.
If you are applying by post, you will receive a letter from UKBA after you have sent in your application, inviting you to book a biometrics appointment at a Post Office. You give your biometrics there, and then UKBA will process the visa.
If you apply in person, your biometrics will be taken on the day, at the in-person appointment.
4) Mine and my wife's situation's pretty straightforward. We both work and have income amounting to £34,500, we're not on any sort of benefits and have the letters and whatnot needed to show we've been living together since I arrived. Does anyone have any advice on whether or not we should be considering the in-person application option?
If you can afford the in-person appointment, I would say go for it. You will get a same-day decision and your ILR visa (the BRP) will arrive in the mail within 10 working days.
Currently, if you apply by post, it's taking about 4 months for the ILR visa to be issued!