We always used the runt of the season whole. I don't see any reason at all you couldn't do the same with either a partial lamb or a big roast.
As far as seasoning started by rubbing it down with a paste of kosher salt, garlic, black pepper, and oregano. With the whole lamb I'd rub the inside cavity down with the same mix, trim off the thin part of the belly and leave it open as there isn't enough meat there to be worth eating and it'll cook faster open to the heat.
Put that on a spit over charcoal and get it started cooking.
In a decent size bowl mix up about half a bottle of a dry red wine. Add about a cup of red wine vinegar and the juice of one or two lemons. To the liquid add about two heads of garlic either pressed or shredded in a food processor, and maybe 1/4 cup of dried oregano.
Lightly baste the lamb every 15 minutes or so with the mixture throughout the cook.
Unfortunately no help on temp or time. It was always a fire that looked right and cook until it was done. I honestly think you could improve it by marinading the lamb in the same mixture as the basting liquid with added salt, and injecting it in the thick parts. I never tried it though. Lamb roast day always started with killing and cleaning the lamb, and then straight on the fire before it even cooled off.