Short Answer: "Two swords will be sufficient" fits the semantics, but has significant contextual difficulties. "Enough!" fits the broader context better, but has other significant difficulties. The best explanation seems to be that Jesus was not thrilled with their interpretation of His instructions, but this wasn't clear to them until after the fact, and Luke wanted to present this misunderstanding to his readers.
Semantics
The word translated "enough" has been well-translated, as the basic meaning of our English word matches the semantic range of the Greek word very well. The other word in the phrase, translated "it is" has also been rendered correctly. So a "wooden" translation of "it is enough" is accurate.
Grammar & Syntax
Possible difficulty with "it is sufficient": If Jesus had meant "the two swords are sufficient", we might have expected "they are" (plural), rather than "it is" (singular), but this is not a deal-breaker.
Possible difficulty with "enough!": In English, "enough" can serve as an exclamation ("Enough!), but it remains to be seen whether this was a common expression in their day as well (as opposed to simply being a modern English idiom which we are reading back into the text.)
Context
This is where the real problems with both views appear.
"Enough!" doesn't fit the immediate context. If Jesus had exclaimed "Enough!" as we might do today, why don't we see any sort of response or change in direction from the disciples? Why don't we see any follow-up rebuke or explanation as He provided elsewhere when they misunderstood Him? It is difficult to imagine that the disciples would have ignored such a forceful rebuke -- especially since Jesus Himself is the One who prompted their decision to go out and get the swords in the first place. Even if they did misunderstand Him, they seem to have at least been trying to obey Him.
"Two swords will be sufficient" doesn't fit the broader context. There are several contextual problems here. First Jesus did not tell them to go find a couple of swords; He told them that every one of them should have a sword, so how could He have then said that two swords were enough? If He meant it literally, they did not bring back enough swords. Secondly, sufficient for what, exactly? Jesus is pretty clear in the hours that follow that His kingdom is not of this world, and would not be won by military action (as we typically think of it) -- in fact, the only thing we see the swords used for is a Peter's lame resistance in the garden, which barely injures one person -- and Jesus immediately reverses this by healing the man back up, and then goes willingly with his captors. What in the world did they need so many swords for (if Jesus did indeed mean it literally)?! Third, aside from the historical question of why they needed swords, there is also the literary issue of why Luke would include a command which wasn't followed, and apparently proved to be pointless except in acting contrary to Jesus. Fourth, if the original instructions were literal, why is it that only the swords are mentioned subsequently? Where is the follow-up on why they needed the other supplies He mentioned? The mention of these original details becomes pointless in the Luke-Acts context, which renders this view highly suspect.
What we see contextually is Jesus first giving a command, which the reader is initially unclear about, whether or not He meant it to be taken literally. (The uncertainty arises because it is an unusual instruction, and because Jesus is often shown to have been misunderstood by those who took Him too literally.) Then we see the disciples taking it literally, but only partially completing it. This is unusual if Jesus meant it literally. (Consider Jesus' acquisition of the donkey or the preparation for passover.) Then Jesus says "It is enough", which is unclear on the surface (as noted above), but is a strange thing to say if He meant His instructions literally (because they hadn't actually obeyed Him!) But He doesn't clearly rebuke them either, which at the very least indicates that He was allowing them to have the two swords. Then we see one of the swords misused, prompting a rebuke from Jesus and a reversal of the violent deed, but aside from that nothing else is said to indicate that His instructions were meant to be taken literally -- in fact all signs point to the contrary, as noted above.
Conclusion
The evidence indicates that Jesus' original instructions were not meant to be taken literally, and that Jesus did not endorse the disciples' eventual use of the swords they procured either. However, it would also seem that Luke intentionally left this unclear in order to aide in his presentation of the misunderstanding. It would also seem that Jesus did knowingly allow them to have the swords despite their misunderstanding, which Luke may have intended as a sign of Jesus' submission to the plan, misunderstandings and all.
In other words, it was necessary for Peter to have a sword in order for things to play out the way they did in the garden, but the only reason he had the sword in the first place is that he misunderstood Jesus (as usual.)
