19-20mm spacers will look spot on..depends on what brand you will go for..had 20mm hubentric spacers and they just slid over the wheel hub...they also were supplied with extended bolts...
I've got 25mm H&R spacers which sit flush sith the body. I'd probably prefer 18-20mm but I prefer the style of the 25mm ones where they bolt to the hub then the wheel bolts to the spacer. I'm not a big fan of extended wheel bolts
My advice is that 20mm are better with stock wheels. The 25's stick out a little too much.
I'd also say that you should fit them all round and not just on the rear. The old cup race cars ran a 5mm spacer on the front and not the rear suggesting that on these cars a wider tracked rear is not ideal.
Are spacers relatively easy to fit? I've looked in the How To section but nothing about spacers. I know with my old car I had to file abit of the spacer off for the bolt to fit.