Let me answer the first question first: My 06 calipers (they are a little more beefier than the 03-05) had about 23-25mm clearance before the FX50 brakes with the stock 20" wheels. I needed 15mm more to clear the 50 brakes. That makes about 40mm total. If I'd guess that the 03-05 calipers are 5mm thinner (I'm totally guessing here, don't know if there's any more lateral clearance than 06-08), that gives a min 35mm clearance to clear OEMs.
Now, as many people have said, the caliper clearance is more dependent on the hub shape and the spoke design. I think you'll need to go with bigger spacers, probably at least 20mm ones (+10 over your current setup), but can't say how much.
On the second question: Just ask for Joe and talk to him about it. The FX50 brakes are the same as the g37 brakes so yes, it is the same upgrade but it's better to talk to someone who knows about it and Joe is the guy..
On the plug&play Q: The fronts are 98% plug and play given that you have the caliper clearance. There are only a couple of bolts that needs to be shortened a bit which can be done easily by a good shop. The bolts are the ones that connect the brake lines to the caliper so it's not unsafe to shorten them, they don't carry any load, just in case you ask...