Err - new automags are about £400 I think depending on what version you get. Check the website at
www.airgun.com for details.
That said, loads of people are selling 2nd hand ones as they have been around a bit and the older styles aren't today's shiny new toy, so you could probably pick one up quite cheaply.
There's a few upgrades you can make, but they work pretty well just as they are. I think the general policy is to stick with company parts, as they tend to work better than other mods by different manufacturers (just what I've heard, feel free to correct me).
As for firing, do you mean if the gun gets fouled by water or firing underwater? If the former, then it's going to take longer for the balls to swell and degrade than it will take to fire them, so depending on the paint and how fragile the shell is, I think you'll be ok. Just give the gun a swab out (my powerfeed classic fieldstrips nice and quickly), or dry fire it to clear any water still in the barrel.
If the latter, then it's going to be an exercise in futility!
Hope this helps.