Looks like you have the correct idea.
The digium TDM04B would be a good choice for you to connect your analog lines to the Asterisk server. It has 4 analog line cards. Just what you need.
The 841 is a good all around phone, and the power supply is very adaptable, so Aussie power jacks should not be an issue.
Things to consider:
You’ll need a power jack for each 841 as well as an ethernet jack everywhere you plan to install a phone. Many phone installations have wiring for a phone, but there’s no power plug nearby. Sadly, the 841 can’t do power over ethernet.
Do you have a fax machine (or any modems) going through your PBX? If so, you’ll want an FXS card to connect it through the Asterisk server or just get a 5th phone line, and leave it off the Asterisk box.
If you’re looking for really good speakerphones, the 841 isn’t it. It’s ok, but not great. They work fairly well in small offices for a single user. Don’t expect to make it a conference room phone. They also have the WORST handset cord. Buy 16 of those as well. They could use some body weight too. They’re small, and light, and I’ve had trouble pulling them around (and off) my desk with the handset cord, so if there’s an option to wall mount them where you plan to use them, I’d shoot for that. Ohewise, I’d attach them to one of these with screws:
ontimesupplies.com/Stands_Te … age_1.html
You’ll need UPS’s for at least 1 or 2 phones and the Asterisk server, or you’ll want to connect an analog phone to your incoming phone lines (turn the ringer off) for loss of power emergencies. If the power goes out, you’ll still want a way to make a call.
Be sure your network is good enough to handle the traffic. Ideally full duplex 100 ports on your switch and on the Asterisk server. That’ll minimize audio quality issues.