To all those wondering, I'm almost positive the downloads will have achievements. I also don't think multiplayer would be *that* hard to implement. After all, plenty of old SNES games, etc, have been available on XBL with not only achievements, but multiplayer as well!
If the game has 2 player support already, online multiplayer is not *that* difficult to implement.
Also consider that if these games are going to costs $15 a piece, then they stand to make a significant profit on something they've already exhausted (I already bought all the old xbox 1 games I wanted by now; haven't you?), so that could be the offset (few extra manhours, few extra dollars). Even if the games did not include a 2-player mode in them, multiplayer implementation isn't as difficult as you'd think (take a fighter, for example). If a game has 2-player mode, then co-op multiplayer is easy as pie. If it has a "versus" mode, multiplayer is even easier to implement.