Anyone know the cheapest way that's not sketchy to do this? Thanks!

The 81-82 maxima brackets are nearly unobtainable, however there are a number of people some on ebay that sell quality steel brackets to use S130 , Maxima or Z31 caliper and rotor. Keep in mind this isn't much of a braking upgrade, the fronts are very inadequate and should be the first thing you upgrade, the rear disk conversion does look a bit better than the drums.

In the hunt for the same mod. Any preference with a specific brand of front AND rear disc upgrades for the 280Z ?

Here si a write up ont he rear disk swap using all nissan parts http://www.zcarcreations.com/howto/rearbrake.htm, The nsisan brackes are NLA you can buy similar on ebay http://www.ebay.com/itm/Datsun-Z-240Z-2 ... 0852415910, or a complete kit http://www.ebay.com/itm/Datsun-Z-240Z-2 ... a9&vxp=mtr

There are a couple of front upgrade options. First you keep the stock rotors and add larger calipers from a 79-85 4x4 toyota truck, this helps if you want to stay with 14" wheels. If you don't mind stepping up to 15's or larger then you can do a vented swap using vented rotors from an 84-85 300zx n/a in conjunction with calipers from an 89-91 4 Runner. all of these swaps should be done with a larger master cylinder, the 79-80 280zx is the ideal donor as it is a bolt on.