70k on an 05 is probably close to normal, considering normal driving will put a vehicle at around 12k miles per year, but that asking price seems high to me as well, but you must also realize that Subarus hold value pretty well.
As far as the heat shield, when standing at the front of the car looking at the engine bay, to the left of the top mounted intercooler and down about six inches is your turbo, it will probably be a rusty cast iron snail under the heat shield, if all bolts are in place, then you should be good to go, many people break a bolt on the right hand hand side of the heat shield(when looking from the front of the vehicle) when trying to remove it.
Check all vacuum lines, they are going to black and about 5mm in diameter(give or take a few mm), and will be all over the place, look for anywhere that may appear shorter than what may hav been stock, they should not be stretched or in awkward positions.
A used WRX always has some inherent risk, but any mildy quick import will have an equal amount, hopefully this is a good deal for you!
Wow, it can turn well too?