Interestingly enough, Hitler himself was somewhat apprehensive about British intervention over Poland. Obviously this isn't to say that Hitler was somehow not committed to the Polish invasion - he most certainly was - but it is worth noting the consideration he gave to the consequences such a move would have vis a vis Britain. I suppose that I am dodging your question in that I don't know of any Nazis who outright opposed the invasion. However, you have touched on an interesting topic within the lead-up to WW2, namely Hitler's attitude toward Britain, which is worth exploring.

Hitler had long desired some kind of understanding with Britain. In November 1933 he had begun working toward an Anglo-German Naval treaty and got it in June 1935 over the opposition of his own foreign ministry and the German navy itself. At the time he insisted that "An Anglo-German combination would be stronger than all the other powers." The British ambassador in Berlin, Sir Eric Phipps, noted that Hitler had "an almost touching solitude for the welfare of the British Empire." On the eve of the Polish invasion Phipps' replacement, Sir Nevile Henderson reported that Hitler had assured him that he "always wanted German-British understanding." One could construe this as apprehension on the part of Hitler concerning a potential war with Britain.

We can of course question Hitler's sincerity in all this. However, with regard to Poland we see evidence that he meant it. When Britain pledged itself to guaranteeing Polish independence Hitler was dismayed, stating that he "couldn't grasp" what they were "really after." Post 1939 Hitler was known to openly doubt "the desirability of the dissolution of the British Empire." In addition we see themes of Anglo-German unity in Nazi propaganda. A briefing note from the Propaganda ministry said that "sooner or later the valuable Germanic element in Britain will have to be brought in to join Germany in struggles of the white race."

The sum of all this is that there is reason to believe that Hitler himself was not initially desirous of war with Britain and that he may well have hoped that the Polish invasion would not bring it about. So while this post unfortunately hasn't pointed to any explicit opponents of the Polish invasion within the Nazi party I hope that it has gone some way to highlighting that Nazi apprehensions regarding a war with Britain over the invasion did exist and did so in the mind of Hitler himself.

Much like one of my previous posts here I have based this answer on the relevant sections of Niall Ferguson's The War of the World. If you want a good read on 20th century conflict then Ferguson's work is a great book to get stuck into.

It is difficult to answer whether there was anybody inside the party's upper echelons who publicly opposed this, however a large number of people were "ordinary members" of the Nazi party, roughly 10% of the whole German population at peak.

There was a loose coalition of government (e.g. Ernst von Weizsäcker), and army officials (e.g. Maj. General Hans Oster, Col. General Ludwig Beck), although only some of these people were members of the Nazi party (as up to 10% of the German population was at peak). These people were divided into two camps, those who simply did not want a war, and those who wanted to remove the Nazis from power. In 1938, they were developing plans, known as the Oster Conspiracy, which would have seen Hitler removed from power had a war broken out over Czechoslovakia.

Poland, specifically, came after this, and at the peak of Hitler's personal power. The Conspirators had been dealt a significant blow by the diplomatic successes of 1938, although they would still go on for the most part to continue plotting to kill and/or replace Hitler through to the end of the war.