You can enter Outland at 58, and there’s one or two quests you can do, though a lot will be grey until you hit 60. I’m pretty sure you can also enter Hellfire Ramparts at 58.
You can either do the Outland starting quests, and just grind, or dungeon run your way to 60 in Outland or level to 60 in Azeroth right now, which will obviously be more efficient, given that you can’t do any Outland content yet.
You can go to Outland at lvl 58 no problem. I started playing during TBC and that’s what I did, and there were more than enough quests to level up without any sort of extra grind required. Granted the mobs were tougher than the ones in Azeroth but no problem other than that.
I am not entirely sure but I think the pre-patch expansion event requires level 60. There is not much content in the TBC pre-patch event compared to other expansion but given this has only been available one week in 2007 it would be could to it this time.
You might as well just do 60 now but if you’re wondering the journey from 58 to 60 in Hellfire Peninsula is extremely short and after that it slows down, it will be among the fastest 2 levels you would have experienced since you left durotar.