Don't think it 100% matters if an idler pulley is a self adjusting tensioner or manual adjusted, both have their good & bad points. An idler inline will also help take the slap out of a long belt length. Street & performance used to be a good source for individual accessories, but heard not the same to deal with since owner died.
I did a vintage air compac unit conversion approx. 10 years ago. Definitely a good thing to have all new stuff. My one wish is that I'd tried to stuff the next size up unit in there, stronger would be nicer. Other problem with after market units, which you won't have is windows fogg up with heat in damp cold & have to run ac full on with heat, but don't need heat that often either. Also wish I'd have done my own lines.
Another suggestion to go along with this is to look into the latest window tints, which reflect much more heat. Heard something about clear ceramic tint that could be used on front windshield, but haven't even thought about it again till reading your thread.