I have a 1991 964 turbo. It is a very low mileage car. It has had a number of modifications including full Ruf suspension, Ruf 18 inch wheels, Ruf short shift kit with "short" gears, 964 normally-aspirated cams, large Andial "Supercar" intercooler, K27 turbo, B&B headers and exhaust. Great car; loads of fun. I am the original owner. I did these modifications over 10 years ago. Car has never had any issues. Key , in my opinion, is to do it right the first time with tried and true components and the very best people to do the work.
If you are planning on staying with the stock CIS injection together with the stock ignition system, I would use the normally aspirated 964 cams and not the earlier SC cams. The SC cams worked great on the 930 turbo but the 964 really light up the 964 turbo. When Porsche built the turbo Supercar for comp purposes, they sourced and used through Porsche Motorsport in the USA ( really a part of Andial) the 964 cams together with the other components in previously described as installed on my car. I acquired the specific parts through assistance from Perfect Power in Libertyville, IL, Porsche Motorsport and my Porsche dealer. The only thing I didn't do wasflow the heads and intake manifold. That's next, along with EFI and a larger turbo to support those changes. But, frankly, I really like the car just as it is; it works great and it has no issues. But, I will stay with the 964 cams. I have used them in this car for ten years. Keep in mind the 91 turbo has an ignition system which impacts maximum boost (essentially .9 bar max), so your package needs to reflect this.
I have heard SC cams are nice but right now just has B&B exhaust.
Was thinking headers, either spring or electronic control for more boost, SSK, software?, etc.