The cam has to come off. No special tools but there's a specific order to loosenging the cam towers. Get it wrong and you'll go from SOHC to DOHC in the negative sense.

Many people have had success quieting the lifters with Mobil 1. I don't know if the success rate is as good or better with Amsoil or anything else. It can take 2-3 synthetic oil changes to to get them quiet and a steady diet to keep them quiet.

The head gaskets are known to disintigrate where the oil goes from the block to the head to pressurize the lifters. Bits of head gasket can lodge in the oil passages leading to lifter noise. Keep this in mind when deciding whether new lifters are worth the expense. I mean if you think a new head gasket is a when rather than an if, maybe synthetic oil for now is the way to go.

I can't tell from the pictures if that's a Garrett or KKK turbo. I'm pretty sure they're interchangeable as units but not as components. Make sure you know what you have before you start buying parts.

Check the parts forum for folks selling 603 turbos or at least 603 turbine housings. I don't know if 617 turbos use the same turbine housing.

A 55 trim compressor housing from a 3.5 liter 603 is an upgrade option. Uses the same turbine section. The compressor inlet is about an inch shorter than the one you have now but that's about the only swap complexity.

Sixto
87 300D