FM immunity - a subject dear to my heart. The silly thing is that the decision to muck about with the frequency spectrum was made over 8 years ago but, like many things in life, everyone ignored it and hoped it would go away. The shocked realisation that it wasn't going to hit the GA civilian community a few months ago and, since then, there have been frantic efforts to get the kit sorted out (at a cost of around £2000 per box with typical Bendix/King kit). Public transport a/c need 2 FM immune VOR/ILS boxes, the rest need just one.
That said, there are no interference problems expected in the UK at the moment and only a few in Europe. I'm surprised that the military have gone overboard on this one as they usually drive a coach and horses through the civilian nav kit requirements. I spent many a happy year crossing Europe with just a TACAN and a 50 KHz spaced VHF and lying to anyone who asked me to go to 123.42.
As far as NDB is concerned, remember that NDBs are just omni-directional transmitters - a bit of wet string would do for an aerial. The problems come from the propagation effects of MF signals and the aircraft equipment - see AP 3456 for details. A typical DOC (protected range) for an NDB is only 20 nms. The aircraft equipment may suffer from dip which causes the needle to fall about 10° to the low wing in a turn. This all goes to make an NDB approach challenging. The only good thing is that, these days, most places have a DME so you don't need to play about with your stopwatch.
A final thought - how many military guys are going to call up for an NDB approach and then follow the ILS indications anyway????