Are we talking about classic Subaru or new age?
Usually buying 5mt DCCD for classic you should also get ecu with loom etc.
It's good idea to get gearbox with rear diff - it should be R180, at least for v5/6 STi's gearbox with dccd - not sure about earliers.
I think that for this diff you'll need also different axles.
In new age - not sure about that - you might already have R180 and proper axles - so putting gearbox, this time 6mt, should be a bit easier.
If you're building stronger car - it might be a good idea to put 6mt new age STI gearbox anyway - no matter if classic or new age car.
The only thing to remember about is, that new age STI gearbox with dccd requires some additional sensors, not present
in classic - and I guess also not present in standard WRX's and STIs without dccd.
This might require aftermarket dccd controller - but, surprisingly, this doesn't need to be too expensive - you can think about Neetronics, which is not too expensive nad pretty powerful.
Anyway the 6mt gearbox itself will be a bit expensive, especially with dccd.
For a street car it might be kind of overkill - but if you build a rally car or something like that - dccd with a good controller is good idea, I'd say.