Are there any limits of re-downloading applications from appstore?
As long as its the same app, then you should be able to do this as many times as needed. Also, once you have that app, you have it even if the price changes, goes from free to paid or vice versa. 

One thing that I have seen happen in the app store, is developers releasing a whole new version of their app, and naming it differently,  launching it differently. A perfect example of it is Tweetie. When they went to version 2, he re-released it as a new app (Tweetie 2), therefore making anyone who wanted to get the new one to have to pay (even if they paid for the first). Then, that app was converted to free and changed to Twitter for iPhone, but if you bought Tweetie 2, you just got an update that changed it.