To oversimplify slightly:
There are two types of head, large and small port.
Large port will, naturally, support greater max air flow, so at the very top end will support higher power. However, small port will flow plenty of air for a 500bhp build and smaller runners mean higher velocities, so will produce more torque. If you're going to be below this sort of figure, then small port is typically recommended.
VVT was fitted to both large and small port heads at one point or other, and varies between engine codes. There was a table out there somewhere with full differences if you wanted to check a particular code. Not a massive issue, as you can swap the VVT/non-VVT tensioners between heads with minimal fuss. VVT can be used to
help with spool a little, particularly on BT builds, so it's up to you whether this is important for your application really. I like it.
Some heads had SAI fitted, though most people remove this as per the numerous guides floating about on the forums. Negligible point, don't worry either way - if your head was machined for it then you can fit a £5 blanking plate to delete this.
Cars fitted with K04s from the factory (AMK, BAM, couple of others) have additional shims beneath the valve springs to
help increase preload a little. These are only ~0.5mm thick from memory, and realistically make little difference. Being an alu head and steel shim, some people like the idea to prevent damage to the head itself, whilst others see it as negligible and not worth bothering about.
Most heads these days are 15-20 years and 100-200k miles old. As a result they can suffer from worn guides, predominantly on the exhaust side. Usual advice when building a head is to replace these along with fitting Supertech Inconel valves (or similar Ferrea equivalent) since old worn valves have been known to break when pushed.
