The horn-hook coupler (Xf2 or NMRA coupler) has been gone for about ten years now. New kits or RTR rolling stock are universally equipped with a Kadee clone knuckle coupler. Rolling stock equipped with the horn hook couplers does still appear at train shows (model trains last forever), in which case, Kadee makes knuckle couplers to fit everything ever made.
As long as you stick to straight track (rails and a tiestrip) they fit. The track plus roadbed thingies from different makers nfortunatly don't fit together.
Actually I have a little of both and they work for me.
Maerklin is a European brand. Most folk do not think of Maerklin when speaking of American HO equipment.
In casual conversation, "power pack" means a straight DC only power pack, where as DCC is spoken of as "DCC". All locomotives will run on straight DC. My recommendation is to begin with straight DC operation and only consider DCC later.
Both sorts mate with snaptrack or flex track.
Different strokes for different folks. The point I was trying to make is that a beginner need not fear buying equipment off the shelf. Buy what ever and take it home and it will run.
David Starr