Based on my strolls through this forum and other research here is what i whould sugest. (ive been looking @ piecing togeather my own turbo too)
The stock BLOCK will handel up to 30lbs of pressure, this is just the casing though. The internals (pistons, conecting rods) might (probably will) give out WAY before that is reached. The amount of boost you want to run is fine and the stock internals will probably hold up for a while with the larger amounts of boost but they will die very fast.
I know Mcnews has pistons and rods the pistons are ~500 and the rods are 360 or 730 depending if you want Eagel rods (350-400HP) or crower bilit rods (almost indestructable). based on your HP goals I would sugest using the Eagles and saving urself a few hundred bucks. if you do decide to get them your engine will be fine till the mid to upper 300's HP range and you wouldnt have any problem running 18 or even 20 PSI daily.
PLEASE note however that the above assumes that you have a good head mated to that block b/c idk how a stock head would react to the high boost.
It sounds like you already have one but if you dont i would sugest a greddy PROFEC B-SPEC2 electronic boost controler (if you can afford it they are abit pricy ($450) to change your boost on the fly to lower amounts or to your maxium tuned amount.
The long and short of the long post is: GO FOR IT!!!!!

they more boost the better. I think that you should be fine and if you change the sugested parts you engine will be vertually bullitproof for the HP you are looking to get from the focus
HTH:
Sarif2029
(ps: try sending tom a PM. If anyone here will have a definate answer its him)