1. Yes, it's been done. Many folks with A4s have done so, and I don't doubt many with 5411s have also.
2. The M25 was offered on many C30s after the A4s and 5411s became somewhat obsolete.
3. The M25 series fits the C30. The M25 itself is no longer produced, but the M25XPA or M25XPB (not sure which, but a friend of mine just replaced his M25XP - with over 5500 hours) with one of those two in his C34) should fit. Another friend replaced his A4 with an M25 he bought on eBay!
4. Things to deal with include the raw water intake to the new heat exchanger (HX) and appropriate muffler, exhaust riser, and exhaust hose.