I need some advice on buying my own plane BEFORE training...

Hello everyone. Let me give you a little background on my situation. I am
25, married, and my wife's father is a CFI at a local field. He has taken
me on a Discovery Flight, and I am hooked. In short, in the past 4-6
months, I have been reading magazines, posts, the internet, etc... all about
flying, but never really got a good answer about what I am looking at.

So here is my situation. I have two options on how to go about my training:

1. I can buy the 5k'ish training from the local company that my
father-in-law works for, use their planes, their ground school, but be
taught by my father-in-law. When all is said in done, I have paid 5-6k for
my PP license, and I have to start renting a plane. I figure it is
~50-60/month to get a loan on the PP license. Continuing to fly, say at
50/hour for rentals at say 6 hours a month will cost me $360/month.

2. My father-in-law has proposed that I purchase a used plane, something
like a Piper Archer or 172 for my training and initial flight hours after
training is complete. He said that in this case, I can use my own plane,
buy fuel, and he will train me in his own time for free. It is not a
non-competition violation since I am related, and we will be using my plane.
I will have to do my own ground schooling, maybe with Kings or Sporty's
training aids. After all is said and done, I will have my own plane to use
at will, and my own license.

What I was hoping for was some advice from the people that have been through
this before. Is it worth it to be to buy my own plane? Say I spent $35k on
a Piper that is in good shape, and assume it has had a recent overhaul.
What can I expect to pay monthly to own a plane?

Here are some estimates I do know. It is 30/month for a tie down at the
local field. It will run about $100-125/month for insurance on average.
What I don't know is what a plane will cost me to finance. Are they done on
15/20/30 year purchases, even though I obvoiusly wouldn't keep it that long?
Is there a "rule of thumb" that can be used to guestimate a round number
for a monthly payment? For instance, it is a general rule that you will pay
$100 for every $5000 you finance over 5 years on a new car... as an
estimate. Can I get some information along those lines on getting a plane?

Sorry to make this so long... it probably has been answered before, but I
can't seem to locate the right info.

Thanks
Anthony