Your single player account is safe.
In your situation, I'd play single player.
You'll learn more through failure, without temptation for a rush, items, etc.
In my experience, you'll want to wait it out for multiplayer.
If you've played vanilla D2, you'll know that a ladder has a lifespan.
That is to say, a finite time window during which trading is active.
Median XL had a very short ladder lifespan last time around.
This content patch is far improved, but I'd say you would arrive to the party a bit late.
Unless you are OK with a lack of trading, but ... then why not SP?
Ladder PROs: Meeting friends >>> Trading > Rushes (once you're experienced)
Ladder CONs: Lag Issues >>>>>>>>>>> Depressing when ladder life over
I'd learn the game in SP, then join The Sin War when next ladder drops
Also, do not assume next ladder = Sigma