1) What are some synonyms for "rude", "boorish", "Uncouth" that you use? Informal words.

2) Does it sound odd to use "all I need is for..." in a ironic way? For example:

(a man is going fishing tomorrow, but it starts to rain) : Just my luck! Now all I need is for this rain to stick around until tomorrow!!"

1) When you say "informal," do you mean slang? Something that you might say to some jerk who did a knucklehead maneuver on the street? ;-)   "Knucklehead" is out-of-date; "jerk" has been around forever, but is still common. Americans, being uncouth in general, tend to use more colorful (Here, colorful means words you might not want to say in front of your mother) language. I can't speak for all people, but many  people in Southern California are comfortable using words like "douche," "douche-bag," and even "jerk-wad."  I believe I've heard all of these terms on TV, so I guess they're acceptable, but VERY informal.

2) "All I need is" is usually ironic, although to use the phrase in your sentence, it's good to add a bigger problem to the first. For example, for the man upset about the rain, he might say "Now all I need is a tornado in the morning."

Another  might be, My car broke down in the  Walmart parking lot. Now all I need is a ticket for unauthorized parking."

Does that make sense? If not, let me know. :-)