How much can I spray and what if any motor work is necessary or recommended?
The general rule of thumb is a 50 shot. Most guys will dial it back a bit and do a 40-45 shot (however the jets work out) to have a better margin of safety. Anything from 50 to 100 will require heavier valve springs as the factory springs won't be able to return quick enough and we all know what that means. I have heard of shots larger than 100 on stock rods, but it's been from shops that go through their motors regularly.
What's the most efficient type of setup?
Cost wise the dry setup is much more efficient. There seems to be a bit of confusion among the internet forums about dry vs. wet setups. You hear crazy things like "a 50 wet shot is way more power than a 50 dry shot". You guys seem like you've done enough preliminary research to push garbage like that aside and for this I am glad. My recommendation? Dry system via spray bar.
Also would I be happier with wet or dry?
Partially see above. Wet systems require ports to be tapped into the head. The only true benefit to a wet system is that you would have the ability to swap your pill size (nitrous jet AND fuel jet therefore HP amount) on the fly with minimal fueling adjustments. Changing a jet size on a dry system requires fueling adjustments via ECU or a PIG. It was never an issue for me as I was already running the maximum size that I could and planned to stay there.
Am I going to need an ignition box?
For a 40-50 shot? No. Run 93 octane on spray days as a buffer against detonation.
Are most dyno tuners competent enough to tune nitrous?
That's a tricky question. I'm going to say no simply for the fact of saving you a headache. Call around and ask very specifically about their nitrous tuning experience. Ask for ET that the customer ran before and after. Ask for HP before and after. Ask what AFR that he finds works the best for nitrous applications. If he stutters at all on any of those just walk away.
Also where is the best place to score medical grade nitrous oxide?
A company called AIRGAS carries it. You'll have to apply for an account but it's not a big deal. If your planning to do you own filling you will also have to invest in a nitrous liquid pump station. While it is possible to fill a small bottle from a mother bottle, it is very difficult to get a mostly liquid fill on bottle pressure alone. The method is to keep the small bottle in an ice chest to create a temperature difference for the liquid to migrate to. Does your local track not have a vendor?
Are you planning on staying stock wheelbase? Either way I'm going to suggest a progressive controller to ramp the spray in. Schnitz makes a very simple two dial controller that simply allows build time adjustment (how long it takes the solenoid to get to full open) and the start percentage (where the solenoid starts as the switch is made). The controller actually pulses the solenoid to achieve these results so it is important to buy a quality solenoid.