I'm thinking of trying out some 20mm-21mm spacers prior to ordering my new wheels

Should I get regular spacers with longer bolts?
Or, Should I get the spacers that have built-in studs and a nut fitment?

Also, If I go down the stud conversion route once i have my new wheels, should the wheel nuts have a spinning washer on them like the OEM bolts do, or are they just a solid one piece nut?

 

Thickness of the spacer would determine the stud setup. I wouldn't do a built in stud setup on any spacer less than 20mm thick just for safety reasons. If it were my car it would be standard studs and slip over spacers.....

Studs with the ball washers are always my recommendation to achieve better load distribution and more accurate torque, and to not gall wheel pocket material. They are not 100% necessary but always the better route.

 

Longer bolts... significantly lighter than the double studs lug nuts setup. 21mm is the limit so you're good!

I suggest H&R brand stuff so you know you're getting premium parts