Steve I'm assuming your ex-demo Aerotec might have cost close to that of a bargain outfit with a lesser inflatable. So I'd echo Max's comment about 15hp min and not spoil it with an underpowered or cheap/old engine. I'd also not waste effort on buying a lesser outboard for the short term as it will probably lose you money and waste effort plus certainly take something away from the pleasure of a great boat.
Max and I have virtually identical Mercury 15 shortshaft 2-strokes in similar superb condition. Picture of mine below (click to enlarge). Shortly before that I had a mint Mariner 15 2-stroke (image below also), not sure if you know these makes/models are effectively identical bar the hood colour/decals.
To give you an idea of values when I sold the Mariner (2001 model) early last year it rushed away within a couple of days of mentioning it on this forum for something over £1200. It was 2 owners inc me, had it's original manual with details of purchase from new and I included a brand new OE prop still in box.
My current Mercury (2004 model) came with boat and transom wheels but the nominal value in the deal was about £1100 and I was happy to pay that for a 2 owner, low hours outboard virtually as new. Look at the prop & skeg, paint not even worn off on the edges.
Now and again someone who's had one of these 15hp motors for years and is out of the loop with their popularity these days under-prices and you get a bargain... but you have to be really quick as they sell almost instantly.
I'd put the Yamaha as an equal to the Mercury/Mariner, the extra weight I think is about 2kg and you may/may not prefer the tiller combined throttle/gears on the Mercury/Mariner compared to the separate gear lever on the Yamaha.
Of course you may want to go larger than a 15 but most folks find a 15 2-stroke the optimum power/weight ratio before they get awkward to move about.
Personally I prefer to buy privately as I do my own maintenance and these 15hp 2-strokes are so simple there's little to go wrong. I also appreciate seeing the owner in a private purchase situation as you can often find out more about their history and maintenance situation. Often if in mint condition they aren't a great deal cheaper privately as we all know what they're worth but it does give you greater choice to find the right one near you.
One dealer I would advise contacting to see what they have in stock or due in soon is Pennine Marine...
Pennine Marine - Zodiac, Suzuki and Yamaha in Northern England
I've had 2 boats and 1 outboard from them with very good service and they seem to have nice condition used items.
Bit of a ramble but hope there's something of interest in there!