i think your target goal is possible. when I was shopping for cars, I saw a lease deal where the payment was around what you want - BUT it had quite a bi tmore down. maybe 4-5k... so it wasn't really 400/month... you can't just look at the monthly for a lease - you have to look at the total costs because many leases require cap cost reductions - which you have to factor in. many lease rates are quoted w/out tax - so you have to add that in depending on your local tax rate.
in general, when you lease you are better off to put nothing down or minimalmoney down. if you lease it, drive it off the lot, and it gets totalled, your insurance iwll pay off the balance due, not what you put down. you lose that.
you may also want to consider what it would cost you to get into a low mileage lightly used certified car. you get a good warranty, and with this market, maybe you can find a nice certified 05+ that would be cheap enough to essentially get you to that monthly price point.
another option is to find a really nice older porsche w/out a warranty. you will pay a heck of a lot less than $400/month, and you will likely get a car that willgive you 99% of the fun of that new one - for a fraction of the cost. bnak the money you would have spent for repairs.
my local dealer sellls warranties through fidelity. my dad had one of those warranties and htey paid on EVERY repair he ever needed. it was not cheap though. i think he paid about $3k for it. at that time the car had low miles. the lowe ryour mileage, the heaper these warranties are - so if you founda great used 986S that you coudl buy for $15k, then you buy one of these "higher end" warranties - you pay another $3k, you sitll will be spending less than 400/month if you financei t long enough.
do homework on warranties. lots of horror stories here of random companies shafting people. don't know what your local pcar dealer sells, but the particular warranty sold by my loal dealer sounds very reputable. the dealer says it's the only warranty company they deal with due to their level of service/history/etc...
good luck. whatever you get new, used, etc - the boxster is a hell of a pile of fun for not much money.
and dont worry about not being abler eto buy it in cash. i can't buy mine in cash either - and like you - i know that my time here on earth is limited, and while i'm not cashing in teh kids savings to buy a Gallardo - i do plan to have SOME fun while I am here - hence I bought a good used 986. and love it!
carpe diem