Another vote for the lay down collar tux shirt and black bow tie if you are buying one. If you own a wing tip collar shirt don't spent the money, but a bow tie is much better than a straight one. And please, a button down shirt does not qualify as a lay down collar tux shirt

Your wing tip shoes will work just fine assuming they are in very good repair and highly polished. If your budget allows, by all means get your self a pair of ghillies. However a good pair of wing tips is preferable to a cheap pair of ghillies.

If you have a day wear sporran you should get an evening one. A day wear sporran is really not appropriate for a black tie. As mentioned above you can get a very reasonable one in rabbit from ebay. They are not fabulous but they will service quite well.

Hose, I am from the other side of the argument. I prefer argyll hose for formal attire but if you can't afford those I prefer off-white. Also these are the easiest to find and therefore the most economical for decent quality. I find black hose too monochromatic for formal wear, and if you don't take my word on it look at Hamish's pictures.