Changing to a triple (low gear 30:27):

Triple crank
new bottom bracket
new long-cage rear derailleur
new left shifter (if the existing one is a double)
new chain
new triple front derailleur

Changing to a compact double (low gear 34:32, maybe 34:34):

New crank
new MTB derailleur
new cassette
new chain

Cheapest option- just a new cassette and rear der (38:34, which is about the same as 30:27):

New MTB rear derailleur
new cassette
new chain

If it were my bike, I'd do the third option, then add a compact crank if it's still not low enough.