I'm not sure what do you mean with "good season" but you could split the year into high and low seasons (or dry and wet), high(dry season) goes from may to october(and in some places well into december) and is the best time to visit the country, only in the amazon jungle the rainy season is important, in the rest of the country (save the coast) it pours only during january - march months, plus, it doesn't rain a lot (only in the andes you'll see heavy rain in these months, say, Cuzco, Huancayo, Puno, etc).
As travelbird said ,the first months of the year are "beach" season in the coast and "rain" season in the andes(highlands), and that's a good time for us to go down to the beach to escape the rain
You should consider booking with a local travel agent as foreigners usually charge more. (and they'll hire a local travel agent anyway)
As anna2009 said, avoid Taca,(i've heard too many of my friends badmouthing them already), LAN may be only a bit better( a lot of my friends badmouth them too) but is probably the "best" you'll find.
Pocket money is good when you are traveling the country as only travel agents and hotels take traveler checks.
To answer your last question, yes, the rainy season is bad enough to care if you are in the andes or the amazon rainforest during it(in the coast you only have high humidity and mist, and when a cat dies, rain), but if you come during the dry season(to the andes) you'll only see blue skies.
If you plan on taking a tour, there are plenty of options to choose from, check
www.totallyperu.com/ and you'll get an idea of usual tour prices for different destinations within our country.
Hopefully i was a bit helpful. Come and visit our country! you won't be disappointed.
And forgive my english, it's not my mother language.
