If you go more than 2 years your rating will lapse and you'll need to do an initial issue test again rather than a renewal. The minimum aids you have to have to just keep a current rating is either NDB or VOR, so you could get away with only doing one if that's the way you wanted to go. If you were going to keep the NDB though I'd probably fly the extra 0.3 and do an ILS as well to keep the lot...