Hi Rodney, welcome.
I'm also building the Tamiya Mk. VIII at the moment, using the Eduard Spitfire Mk. VIII exterior set (32664), WIP build is here in the Pacific War Group Build. I guess this exterior set is included in the Big Ed. It's quite nice and adds some detail. Rolls Royce labels for the Merlin is the best bit so far!
Of course the Tamiya kit is so beautiful adding ALL the Eduard bits is optional. It's really up to you. I think that some kits 'need' aftermarket to establish a reasonable level of detail, but for the Tamiya kit it's just icing on the cake.
As Erik said - the main thing is - enjoy.
cheers, Tim