if upgrading, then its a balance.. of initial cost (if upgrade is likely then first considerations for manifold etc is key)
If you choose the popular IHI JS type approach you have the benefit of it being the most popular BT upgrade in the UK. All courtesy of Jabbasports initial developments. A solid 300-320bhp its a good starting point. Now.. if you always plan on evolving things, doing rebore, adding some forged internals, valves, springs etc etc.. considering bigger turbos etc, then that initial choice of manifold, downpipe would now be a significant factor/limitor.
top mount vs bottom mount etc.. being an example.
Using the JS example, IHI VF34 or equivalent TD0x unit, the size limit will be determined by what will fit under there comp housing wise. Owens unit is a heafty power packer, well designed, and I've heard is flowing >500bhp on a modifed manifold(external gated added)
Balance of mods and the first rungs of the ladder you wish to tread. You may have to go back to go forward again if you choose the wrong start point.