It's best not to have gravel or a substrate in a quarantine tank, because if your new fish do bring in a disease you will have to sterilise the tank, and this either means throwing away or boiling the gravel - easier to have a bare bottomed tank.
With regard to cycling, keep an extra small filter running in your main tank - when you use your quarantine tank just transfer the filter over - instantly cycled. If you need to sterilise it either throw the filter media away or boil/disinfect them. Or use the same sort of filter as your main tank, and just steal some filter media from that when you need to use your quarantine tank.
You don't need to keep your quarantine tank set up all the time. When you get new fish just get it out, fill with the right temp water, put the heater and pre-cycled filter or filter media in, and you're set! Don't have too many decorations, maybe a plastic plant or two and a clay pot - anything that can be easily sterilised if necessary, just enough so the new fish have something to hide in. Also, by not having too many decorations it's easier to keep an eye on your fish, and by having a bare bottomed tank it's easier to keep an eye on their poo to check for parasites/infection etc.
HTH!!!