Keep the ECU as a spare. So your car was a light pressure turbo to begin its life? Yes, the 'new' ECU is required to get the extra boost, the stock injectors would have worked. Be careful with 'full' boost, as the internal engine items are not designed to handle too much torque. You don't want a wrist pin breaking.

If the boost gauge was mounted on the A pillar with a vacuum hose routed to the vacuum tree by the throttle, mount it again as a nice to know item.

As for the radio, there are numerous after market double DIN replacements in any price range. Just make sure you get a proper wiring harness to mate with the 1998 wiring. I would have just opted for an original radio from Ebay.

Body strut??? Do you mean the bar that connects the two strut towers? If it doesn't get in the way of the brake fluid container or make top engine maintenance a real pain, just mount the thing and get less body flex when cornering.

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Keeping it running is better than buying new