I'd get the whole engine. VVT engines came with better oil pumps (needed to supply additional oil to the VVT mechanism), higher compression pistons and some bottom-end improvements (i.e., the oil pans won't swap because of an added girdle). This is assuming you were just planning to drop in a used engine. If you're planning some kind of monster rebuild, then never mind . . . .

lets just put a that inyo perspective. this is a N/A situation
lets just assume for a moment your 94 head had the same flow characteristics as the VVT,
old scool would just put bigger cams into it.get it ported. cause its all about top hp.right?

the problem with the old scool way is that if its a road car. with big cams, youre going to loose a bit of driveability depending on how agressive you go.
lets say you go somewhere like 272 cams, not over the top, but enough difference to make it noticeable. it would certainly make the car more dramatic. Some people like that.and thats OK.
The NA8 (is it BP5W?) has cams of around 235/250 advertised
its a bit more agressive that the B6 but very livable smooth idle & very economical fuel consumption.

the bigger the cams duration & overlap is, the more power you get at the top. Overlap is all about scavenging at top RPM. It makes the air race through much quicker. However with old school cams, when the overlap is fixed and it makes idle a little more lumpy at idle. You will loose some low tomidrange torque & power. at idle big cams use a shit load of fuel. Cause there is too much disturbance in the airflow in low rpm. Which is no drama in a race car but in a road car. It should be avoided. Thatrs why they have consevative cam specs so the motor idles & has good fuel economy.

enter VVT. Theyve actually go pretty agressive cams. However because the cams duration & overlap is adjustable. you get the best of both worlds
smooth Idle cause the cam is adjusted to a conservative spot and more agressive at high RPM where you want it.

It gives you more midrange power. for the same cams in a figure of speech. Better idle & better fuel economy and the same top power.

that alone for me would be worth a lot.
a set of cams is going to cost you the same as the VVT Head, then reality is that the VVT is better flowing. yes It will be more work.

now the crux. If your going turbo. They dont lke scavenging as much....maybe someone with a little more knowledge can tune in for that.

and yes....Ive simplified this dramatically.

One reason to use the 94 block over a total 2001+ engine might be if you wanted the handy turbo oil fee below the exhaust manifold. But you can get the feed from the front of the VVT assembly. There is documentation on that to be found right here on MT.net.