Scirocco Register

Question.

Current car, GTX 1.8 8v injection. I'm wanting to add 4 spot lights, a newer headunit, a decent set of speakers, extra gauges, some additional interior lighting, satnav built into the dash and other little gubbins to play with.

Running at the minute with the lights on, the fan, the stereo and the wipers it sits at around 11.5v, maybe a little under. With stuff off its a healthy 13v.

Will I need to increase the power output, be it alternator and/or battery, to reliablly run the new kit?

If so, to what?

I belive the later/last Sciroccos and late Golf cabriolets came with a 90 amp alternator as standard to help with the extra electrics, I guess like me you have a 55 amp alternator?

Yes, the later cars had 90a alternators. Easy to source as widely used across the VW range.
I uprated both mine to 90a for the same reasons.

Start with checking voltage drop on main earth leads and alternator +ve cable