Hi Regan,
M54B30 engines seem to be thinning out a bit lately. There's a few wanted ads on here for them. The ones I've seen come up on Tardeme recently have been quite pricey at around $3k complete. Realistically, they're probably worth around $1K - $1.5K, at least that's what I've paid.
Check my thread. I'm going down a similar route - aiming to incorporate aftermarket parts where I feel they make a good upgrade from factory composites, etc.
Things amassed so far include, steel pullies (waterpump and powersteer), braided vanos line, alimunium thermostat housing, alimunium cam cover, etc. I'm also swapping to an M50B25 inlet manifold to delete the disa,ccv, etc (I have a 'Seems Legit', {legitimately expensive} conversion kit for that, but will have to make my own hose setup as I didn't want to remortgage).
Most of the bits I've bought have had to come from overseas (ECS mostly) with the exception of the Chase Bays vanos line which if I recall was economical to buy locally. The other bits were cheaper offshore, even when factoring in USD$100 shipping per order.
Don't forget the clutch kit if you're going manual. Plenty of choice in clutches locally, but I sourced all the bushes, etc from the US again.