The month of May just ended (haven't built hyperwave relay yet) and a nice influx of cash has allowed me to have 6 satellites in the pipeline (20-11 range of days). My current satellites stand at 6/10 so I'm going to need to put up my final satellite nexus as soon as I get the money and materials to put it in. My first firestorm will be completed in about 7 days, but I don't know how many of them I'm going to need to build to cover the new continents. Will I be able to quickly transfer them over to whatever area gets a large UFO or am I going to need one in each continent? Will they replace all of my interceptors or should I have a mix?

On a side note Mutons just arrived and I haven't really bought upgraded equipment. I could build the Foundry, but what would I even buy? It probably would be better to buy the armor and weapons upgrades available, but I don't know if its worth the money. Any advice on that?

Transfer time is too long, UFO will get away. So you will need 1 on each continent, if you want a guaranteed coverage.

You can trash all interceptors afterwards. A firestorm with either plasma or emp weaponry can take down anything.

On normal or below nothing in the foundry is crucial. There are some things that might make your life a bit easier, like better scope, better arc thrower, alien grenades, improved medkit, ammo conservation (arguably the best of the upgrades).

For Foundry stuff, I've started really liking the SHIV. It's useful for two reasons: one, it gives you a scout to send in which is expendable (can always build another SHIV), and actually effective (unlike a rookie). Two, it's useful to bring one in situations such as a UFO where you know you'll be facing sectoid commanders, which typically like to mind control something as soon as they see it. They won't be able to do that with your SHIV, so it's safe to send that thing in to flush the sectoids out.

The downside is that when you bring it, that's one slot that you can't have a soldier who is gaining experience. But it's still very useful even with that drawback.

Kind of answered above but for normal: have one on each continent with an EMP. You will not be able to transfer and catch UFOs and leaving one to go wild will really hurt you with a given country.

The Foundry is worth it for ammo conservation alone, so do build that. I also did like the pistol upgrades later on as they helped my two sniper units to be effective on the move (as I set them up without snap shot).