On the stock heads, you may see 10HP. To install them you will need an elbow for your throttle body and your stock strut brace will not fit. $200 isn't a bad price.
Never never never quote a shipped price unless it fits in a fixed rate USPS box or you weigh it and get an exact number. FYI you may have been able to buy a box and ship everything for close to the same price. Always tell someone if there is a problem, don't just ship it.
Get Hawk pads. Don't get drilled rotors, they will crack. Slotted are mostly okay. Regular Brembo rotors are fine unless you are doing autocross. Don't buy cheapo rotors.
That's a drop in the bucket of the full cost of the conversion. A full kit will cost a minimum of $1500+. Other than the trans, you will need a new computer, clutch, flywheel, peddles, shifter, cables, etc. The labor to do the switch is significant.
That's a huge leak. You may have a cracked housing or tail. If they say you need a new trans, you may be better off getting a used one at a bone yard and having that one rebuilt and installed. To do a rebuild, many places charge $800-$1200. When mine goes I'm ugrading to a PMA-PA45101-50 $1600...
You won't get 350hp with GT40 heads. I don't know if you are wanting 350hp at the wheels, but to get those numbers you will need to get much better heads, a blower, or stroker. GT40's with a mild cam will get you about 250hp at the wheels.
You need to clean and flush the block and radiator. You need to get rid of rust and gunk in the system. If you don't clean and flush it, the new core will corrode and leak very quick.