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The stock stereo uses a common ground on the speaker output (same for the amp input). Modern stereos use independent grounds for each speaker (floating ground).

The only head unit-to-amp interface you'd need is a FGA (floating ground adapter) which costs $5-$10, not $249. Sounds like the shop is imposing Porsche tax.

I'd replace it all (HU, amp and speakers), but if you're happy with the way the amp and speaker combo sounds, all you need is the HU and FGA.

The stock amp only outputs about 25watts per channel RMS, so most HUs with built in amplifiers are more powerful @ 45-55watts per channel. If the new HU you choose puts out more than 25watts per channel, ditch the amp completely as it will just choke the power (like feeding a 1" plumbing pipe into a 1/2" pipe).
If you replace or toss the amp, you need to consider the speaker ohms vs the new HU and/or amps ohm load capabilities.