dave you can do a few things each with it's own for and against's.
1. cheapest re-use factory manifold.
2. find a 2nd hand factory manifold and get it extrude honed and it will flow much more than standard which should lower the boost threshold and lower exhaust manifold pressure = more hp earlier. cost of honing $600 ish. bolts straight back up.
3. HKS cast manifold which flows much better than factory (prob equivalent to extrude honed factory unit) but you will need to modify the dump. you also have a port for an ext gate if you wish to go that way. extrusion of this manifold also has gains. unlikely to warp or crack since it is a cast unit. manifold cost maybe $600-$700 probably with old HKS gate attached. some would say the capacity to absorb/retain heat makes these one of the best options. check legend01 effort with an RB2.1 HKS mani extrude honed + GT3040 with both turbine and comp cover extrude honed....amazing stuff. 306rwkw with around 250rwkw from 4500rpm (or thereabouts out of an RB20 engine with small valves etc). plus the HKS unit is divided should you want to look at twin scroll turbos for earlier spool.
4. aftermarket manifold like 6Boost etc probably $1000-1200 nicest design, possibly greatest hp potential but again exhaust modifications to fit.
5. cheap chinese copy = possibly some risk of cracking later.