I would go with a front mounted intercooler if I was you, Darkside products are very good but a bit on the expensive side. Depending on your target a slight upgrade of the bv43 turbocharger will release a bit of power with improved driveability- I can upgrade your turbocharger with a billet compressor wheel which is lighter and has a better trim so it spools quicker and flows better up the rev range. Your limitations are the injectors, there is no upgrade for the Siemens PPD injectors so only solution is to revert to pd140 Bhp Bosch injectors and ECU, it's a rather simple upgrade which opens a whole new page in tuning possibilities with up to +200% fuel increase if aftermarket injector nozzles are used. Where abouts in the country are you ?
If you want to go custom I have developed a custom tubular manifold which will take a t25 style turbo- they are easy to upgrade and readily available and will flow enough air for up to 300 Bhp with just a cartridge upgrade and machining. You get modern compressor wheel technology and staggered gap oil seals for long life... Please get in touch with your requests, I have been playing with turbos for over 10 years now so I do know what works and what doesn't ! The last pd170 project I worked on we fitted a gtb2260vklr ball bearing jobbie with custom tubular manifold and it did over 300 Bhp however EGT are the main concern... Regards,
Chris