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Even budget legacy decks, which is what you would expect at a "semi-casual legacy tournament" as you call it, will kill you in 5 turns or less if you don't seriously mess with their gameplan, some much faster like budget dredge which has a very consistent turn 3 kill.

You'll be facing

Burn decks (turn 4-5 kill)
Elves (probably before turn 4)
Enchantress
UR Delver
MUD
Mono red Sneak Attack
Dredge (very fast and very cheap also milling them will help them)
Storm (ANT and TES)
Death and Taxes and Maverick
Rogue decks like Hulk Combo, Cephalid Breakfast (combo decks that will kill you very fast and milling them helps them)
Goblins (turn 3-4 kill)
Solidarity
Pox (pretty slow but very disruptive)
Merfolk (very fast and able to counter key spells)
Budget Loam (milling them will help them)
...

basically all pretty cheap decks (some pack some expensive cards but a lot of legacy players still have these cards from when they were cheap) but very deadly and very fast. Basically you need to be able to interact with this format as of turn 1. You can't play spells like Cancel or Consuming Abberation, they have no effect. It's way to slow.
Not trying to break your spirit but do be aware that legacy is a very mature format with a lot of very good veteran players. Even now that everything has become so expensive lots of legacy players already have most of their expensive old cards which used to be dirt cheap or can still build a budget deck that when played well can perform very well when not everyone is playing Tier 1 decks.