Here's something you wouldn't expect.
Whether this applies to all counter stern boats or not, I do not know.
I had a Yachting World Diamond once. (It was a cruising version, ie someone had added a cabin and a bit of accommodation). It had a fin keel and a separate rudder, no skeg. The rudder had a considerable balance and was set fairly well inboard from the transom. (I never liked the rudder much, but that's another story.)
On the end of the long counter, on the centre line and in the slipstream of the rudder, was mounted an outboard motor on the usual fixed bracket. About as simple an installation as you could imagine. If I recall correctly it was a 9hp Johnson, standard shaft. You had to crawl out onto the long narrow after deck and kneel, while leaning over the stern, to get the thing started. Pull start/skin knuckles arrangement.
Here's what happened. To my amazement, it steered by itself. I had visions of having to squat on the after deck trying to steer the wretched thing, with the boat tiller 2 metres away and out of reach. But it turned out that it was possible to sit in the comfort of the cockpit and just steer with the boat tiller in the ordinary way. By some fortunate ordination of Providence, the outboard motor simply followed the attitude of the rudder. If I pushed the helm over, the motor would turn in sympathy - and add positivity to the steering. Being mounted on the centre line, it was never bothered much by the boat rolling or heeling. (Mind you, I never used it while sailing - and as for reverse, I don't remember now, but don't expect it was much good for that.)
The point is though, if you just want a simple stern mounted "kicker" I think you can put it anywhere you like on the transom (away from direct interference with the rudder) and if it can swing freely it may well just follow the slipstream and steer "with you" by itself, as mine did all those years ago. I'd put it directly behind the rudder, if that is possible.
I've had a lot of different outboards on a variety of boats over the years - with varying degrees of success and satisfaction/frustration, and I've seen more than a few busted pull-start mechanisms and oiled up spark plugs. My most recently purchased was a 9.9hp 4-stroke Yamaha Hi Thrust. I did a 5-day river trip with it. With electric start and electric tilt. What luxury! It's worth it, if you must have an outboard. Arne is right about that. It has a feeble little charging plant, but enough to put back into the battery what you use when starting up. It's quiet and it's a gem. Its the quietest and smoothest machine I have ever owned (with the exception of my sewing machine). Its the first time in my life I've ever loved an outboard motor, and there's been plenty I have hated.
You see these Yamaha 4-strokes for sale from time to time second hand, usually with a history of being the back-up motor for some rich person's gin palace - sometimes with hardly any hours on the clock. Right now there's one on Trademe: long shaft, electric everything, Hi Thrust (that means bigger propeller and slower rotation - what you want for an auxiliary) - remote controls so you don't need to lean out over the back to fool around with the thing - with all those extras its starting price is NZ$1,990 and nobody has bid on it - it's come back on Trademe for the second time. It's actually 8hp - I've never seen one of them before but it looks just like a 9.9 and I doubt if there would be much difference.
You'll find it here. It might just be what someone's looking for.
The remote control and cable is probably worth $500 on its own, and if you've had one - its an order of magnitude better than facing backwards and needing to use two hands fooling around with tiller-mounted controls.
PS At one stage I had a 9.9 Yamaha Hi-thrust, and a 15hp 4-stroke Honda at the same time - also a nice motor with all the same bells and whistles. I was able to try them both out on the same boat. The Honda was marginally faster in a straight line but when the non-planing hull went into a turn, at full power the Honda propeller would ventilate and slip (what some people mistakenly call "cavitate"). Horse power is no use if you can't convert it efficiently into thrust. The Yamaha was quieter and all-round performed better for this particular boat. I had the choice of which one to keep, and chose the Yamaha without hesitation.