You can put rings on most spacers now. Most of them come out at 5mm, 10mm, 15mm, 25mm and 30mm. The rest are either uncommon or custom. I'd go custom so you don't have to buy rings anymore, although if you decide to part ways with your spacers it's a bit hard to sell.
You'll need longer studs if you go with sandwiches instead of adaptors. It's a must. Mine's all custom from
Fred Goeske. Call him and ask him if he can make you one for your car. His spacers/adaptors are very durable. I used them in the track and I was running 15mm sandwich up front and 35mm at the back.
I recommend less than 10mm if you're going sandwich. You should ask around in other forums too. Because mine is all from experience. It might differ from other drivers.