Nos. 1 and 2 will be similar to your DTV experience. You already know how it behaves in wind and rain - you can expect the same for D-PC. No. 3 - you've already got one dish in place, why rip it out if you don't have to? There no particular advantage to two dishes, so unless you have some aesthetic preference for one, I'd say don't bother.
4) depends on the phone company. If you call them, it's very likely they'll tell you you're too far and you're out of luck. And that'll be the end of that.
5) People have alluded to the latency of the connection already. I'll tell you straight up - you can forget about online real-time gaming with a satellite connection. It just ain't gonna work - the latency is waaaayyyyy too high. You can try, but I promise you won't find the results useable.
6) Get a hardware firewall/router and plug the satellite modem into that, and pass the connection through it to the computers. You'll sleep a lot better at night, trust me. ;)
Last of all, I assume you're aware of the price of all this - Directway ain't cheap. And the other thing you should consider is that Hughes has serious, hard transfer limits. If all you do is websurfing and email, mostly, you'll be fine. But if you do a lot of filesharing, or other large downloads regularly, you're going to hit that cap REAL fast. I suggest you take a good, hard look at D-PC's Fair Access Policy to see if that's something you can live with...
The gaming is for my son, so it doesn't look like he would be to thrilled about it.
Do you know why DSL over the telephone lines are only good up to about 3 or 3 1/2 miles from the CO? It kills me that we happen to be a little over 4 miles out at our place up in the mountains. What is it, signal loss of some type?