Excellent story and it doesn't get any better than a military issued SM300. Those things are on a par with the Milsub in rarity terms, although a little behind price wise. Someone got a great deal buying that watch, if you look on Chrono24 etc most are north of £50k.
A bezel and the correct insert shouldn't be too hard to find and after a good clean up that watch will look superb. The money is in that dial and case back.
I don't think the dial is relumed. A lot of military issued SM300s are that colour and the all important pin pricks at 12 and 6 look good. I think that is an untouched dial.