I wouldn't use POR-15 in the midwest... I've spend hundreds of $$ on it with little success. The problem is that it peals, and if there is even a hint of moisture under it when applied, you're hosed.

What I would do is the following:
Coat the inside of the frame with a nice thick coat of used oil.
Have the wheel-wells Rhindo Lined or Line-X'd (Do not use a home-installed liner). Have the interior of the wheel-wells / quarter panels lined to seal all seams.
For the undercarriage, I'd clean up everything and spray it with a good epoxy primer, sealer and top coat of chassis black.

Then find a car wash that sells unlimited washes (usually $30-40/month) and take advantage of it.