While it's true that the Honda 1800 has a lot of power and also a reverse, both of which make it popular for trike service, they do have some problems.
AFAIK, they have been plagued with cracks in the aluminum frames pretty much in all years. That would make me think twice about doing a trike job on one of them, let alone buying one for solo use. The pictures of the cracks have been all over the internet and also Honda did institute a recall which was pretty much a bandaid to weld the ones that they found cracked with TIG. The originals are MIG welded robotically on the assembly line.
I think a T.C. with a 95 inch engine and a mild cam would be a pretty good trike machine. Bear in mnd that you are adding about 250 to 300 # and also the gearing will be somewhat lower.
I've ridden several trikes and they do handle differently but, kept to a decent speed, they handle pretty well with the understanding that there are limits on cornering, much like a sidecar.
The only deficits with a Harley would be the lack of a reverse gear and also air-cooling with a greater load imposed. It's possible that some of the companies may have a reverse but I'm not sure. I know that there are aftermarket reverse gears as add-ons for HD transmissions but they are pretty expensive, too.
In the years to come, some of us may be getting a little older

and getting a trike is one way to stay "in the wind". I would think that they would be a fun project in any case.
As to cost, it was mentioned above that Motor Trike is about $8,000 for the kit, exclusive of labor, and I checked on Lehman at the Laughlin show a couple of years ago and they are about $10,000 for a Harley Trike, plus labor. That makes any of them a pricey deal for sure so you would really have to be sure that this is the way you want to go.
I also think that doing a trike would void any warranty that you might have on either a Goldwing or a Harley so that could be a big consideration unless the bike to be utilized were out of warranty.
My 2 centavos.