Epoch- that is, unless you can somehow quickly gain it without having to trash more than 1 card or so, which seems exceedingly unlikely.
I love the gardens, and go for them at basically every reasonable opportunity. On a strong Gardens board, you want to start getting gardens earlier than you otherwise would, say with 2 workshops on turn 3 (quite possibly the best play). On weaker boards it can often be helpful to have a flexible opening, as you need to guard against your opponents either trying to gardens OR go provinces. Games with Plat/colony make it SLIGHTLY harder to go gardens, but not much - you're usually too fast.
Watch out for really good acceleration by a player going for a different VP strategy, because if you're outpaced, you're lost.
Bureaucrat is one of the stronger enablers, but while you're going 'pure gardens', you aren't going 'classic gardens' and rushing three piles.
Ironworks is the number one enabler. If Ironworks is on the board, you MUST go some variation of Ironworks/Gardens.
Next is probably bridge, which helps run estates out quickly, closely followed by Horse traders, workshop, and woodcutter. Actually Horse traders might even be better than bridge. Hamlet and pawn are also really good. Worker's village less so. Talisman is middling.
Bishop is the Gardener's bane, and only the strongest gardens boards will work when bishop is out.