If you want straight line performance or more than 220-250 whp without forging the motor, walk away. If you want something to take down those pesky miatas in the twisties you found your car...
Ive seen sheetmetal fender kits online somewhere - I live in MN also... and my rear fenders are looking BAD.
The rust is coming from the inside - there is a spot that traps water(and salt) inside the fender and once it starts...
If it isnt to that point yet - I would Undercoat the S*^T out of it and keep it waxed. If my car werent so far gone I would also peel back the carpet and soundproof the fender wells as there is a LOT of road noise coming from there you can hear every speck of dirt bouncing off the tires.
Welcome! You've probably found out already that these things handle really well out of the box! Though suspension is always the wisest mod!