「私が ”変なおじさん"です。」I am the queer guy (pervert). the pervert you know through the media, by rumors or whatever.
「私は ”変なおじさん"です。」I am a queer guy (pervert). one of perverts
「今が/今こそ (この冷蔵庫を買う)チャンスです。」Now is the chance to buy this fridge. You have no more chance.
「今は/今(この冷蔵庫を買う)チャンスです。」Now is a chance to buy this fridge. Now is one of the chances to buy this fridge. You may have another chance.