Sweet - I'm down for keeping it simple. AVCHD may be a tiny bit lower bitrate, but will still look awesome :)

As for lossless audio: consider stereo (2 tracks) 16-bit 44.1Khz WAV format about 10MB/min.
FLAC is about 1/2 that, give or take. At, let's say 121 min., 1 stereo track would be about 605MB. Half that for one track, then times six (for "5.1") = 1815MB.
If FLAC isn't used, and we're talking 16-bit 44.1Khz uncompressed, 3630MB.
If we're talking 24-bit 96Khz WAV, likely around 6GB. At that size, however, you'd still have room for most, if not all of the other "lossy" tracks I would bet, plus a relatively small animated menu. Might be pushing the limit of a 25GB ("23.3" usable is what shows in Windows...) disc, but it should end up being "just right." (6GB + 16.5 = 22.5 .. little room there for a menu still!).