I suppose it's up to how you want it doing really. I have a Mk1 with the roof plated. I removed all of the fittings and was left with the plate that was the sunroof. From the inside welded strips on all 4 sides, silicone sealed it and tank taped the outside and inside. I've just replaced the silicone after 5 years as got a tiny drip now and again in torrential rain....but nothing now re-sealed (there is a lot of stress on the car on track as you might imagine)
The other option to plate can be expensive as they need to cut the alloy, seam weld it, grind off, check the welds, and paint the roof (they can't use any filler as the stresses will crack it straight away, and if they recomend this walk away as they don't know what they are on about!)
To do what I did will take them 1/2 an hor to weld well...so 1/2 an hours labour(£50 tops) tank tape (£5.99) silicone (£4.99) so all done for under £65........or at least £300+ with the alloy plate option
