According to a post by a Blizzard employee, you might be able to play with your friends no matter where they are:
Getting people online, playing and interacting is obviously the overall goal for the Battle.net platform, and that includes allowing people to play across regional boundaries as they have in the past.
Unfortunately, there are a multitude of challenges we have to overcome due to the unique regional account and billing options that didn't exist in the past. But those hurdles aren't insurmountable, and we are looking into solutions that will allow interested players to obtain access to other regional versions without having to buy another full copy of the game. Those solutions are something we're currently planning to have available through Battle.net Account Management within the first few months of StarCraft II's release.
Before that solution is implemented though, you're correct in that you'd need to purchase a US copy of the game on launch day to play in the US region.
However notice that he is talking about a potential future feature which may end up not getting implemented (or not implemented in a satisfying way), so in the meanwhile I would be careful about which version to buy.
Do you want to know how someone can lose or how he did lose?
Bly lost because he played very poorly: he did not build enough drones.
Take a moment to let that sink in, because its a very very big mistake and its the difference between a good Zerg player (like Fruit Seller) and a bad Zerg player (like Bly).
Instead of backing off after taking the Terran Natural, he continued to press in. He sends wave after wave of a consistently sized army. He does this because he's not building more drones so he cannot afford a larger army. By comparison Tarson's army gets larger and larger after every wave. He's able to continue to accumulate units because he has a fairly healthy economy. This is due in part to MULEs, but largely because Tarson built enough SCVs. Even after losing his Expo he still has as many workers as Bly does. Think about that for a second. He lose an entire base and all his workers in it and he still has more workers than his opponent.
This game could have easily been one that was featured on Day 9's Newbie Tuesday
I could point out a dozen mistakes that Bly made, but none of them matter nearly as much as his poor drone usage. Because of his lack of drones, his assault becomes almost an all in. So in response to your question How is it possible Zerg with biggest advantage can still loose to Terran? Bly didn't have a big advantage. He sacrificed his economy on a gamble and it didn't pay off.
But let's imagine another universe, where Tarson wasn't playing Bly but was playing against a better Zerg like IdrA; and let's assume IdrA made the same early push.
Now IdrA is sitting on 2 bases to Tarson's 1, but they have equal workers. So instead of continually pressing his advantage, IdrA decides to play like he always does: Macro. He sits back and takes a third base and powers drones. Tarson retakes his natural and now he's down 2 to 3 bases. Not a big advantage for IdrA, but remember a Zerg can make drones faster than a Terran, so it takes Tarson longer to saturate that 1 base than it does for IdrA to saturate his 3. Now IdrA has a huge economy and he goes Roach/Hydra (or these days he's favoring Muta/ling, but pretend).
Imagine that battle for a second... or don't, because it actually happened. IdrA knocked out Tarson in IEM, exactly like I described.
The smartest thing you can do in Starcraft if you are ahead is to get more ahead. Anyone who played seriously in Broodwars knows this because if you pressed like Bly did, you got destroyed 100% of the time. Now Starcraft 2 is still new so a lot of people feel like they can do crazy 2 base play, but if you look at the high end GSL matches you see less of that and more of people trying to get more ahead.
You also mentioned a quote talking about Marauders vs Banelings. Now I don't think that the relationship between these two units had anything to do with this loss, but let's talk about them for a second.
Banelings are a highly specialized unit. Their role is to act as Zerg splash damage on light units. By comparison the Marauder is a more generalized unit, designed to act as a Meat shield (much the way the Roach acts for the Zerg). So it makes sense that a Baneling doesn't function well against Marauders (as that's not their role), and it also makes sense that Marauders do a good job of preventing Banelings from striking other units (as that's their role).
I would hesitate to say that a unit is broken because its functioning in its role and another unit isn't functioning in something outside its role. But let's say you want a way to deal with MMM balls. Instead of sending it waves of Banelings on their own. Consider mixing in more Zerglings (Bly actually does this to large effect). Now let's vary our composition based on their unit mix. More Marines? More Banelings. More Marauders? More Zerglings. Can't find a good place to engage? Burrow/Drops/Fungal Growth/Wait and Flank.
Here is the key thing, none of this is any assessment of Banelings against Terran, but rather an assessment of the tactics used.
Best Answer
First of all: LOTV is a stand-alone game, you dont need the previous 2 games to play it. That might resolve your question alreay. If not, read further.
Every expansion has it's own matchmaking, so if you only have HOTS, you will get matched vs other HOTS players. The same goes for WOL.
There are still people playing HOTS and LOTV as stated in this reddit thread, so if you only want to get one of those, you should be fine.
But this thread is 6 months old, so things might have shifted a bit. WOL has probably lost a lot more players with the release of LOTV. You should be fine for lower levels as far as queue times go, but most of the higher skilled player have probabably moved to LOTV, so I expect queue times to be quite long after platinum.
HOTS however should be fine, since LOTV is only out for less than 6 months.
If you choose to get only WOL ot HOTS, you should also keep the following points in mind:
My recommendation:
Get the starter edition first. Starcraft is a hard game and not for everyone. If you tried it out and like it, I would strongly recommend getting LOTV. As mentioned, you dont need to buy WOL and HOTS for that. I wouldn't really recommend getting only WOL and HOTS, since both together are the same cost as LOTV.