In my world the Cincinnati would be the size of choice. It's big enough, sturdy enough, and powerful enough to handle any job I'm willing to do. The larger work envelope allows the tailstock to be moved far enough out of the way so as not to interfere with the operator or various tooling when not in use. A drill chuck, center or other tooling can be left in the tailstock without interfering with the operation at hand. It can then quickly be brought into position to drill ream, counterbore, countersink, or perform other operations.
The larger envelope for the chuck, and larger spindle through hole are also a plus. I can't count the number of times I've had to do work arounds on smaller machines because limited chuck or through spindle capacity. When I first got my Seneca Falls 10" machine I noticed the bed had been relieved close to the spindle. I didn't think much about it until one day when I needed to install the 8" chuck. It was then I found the relieved sections coincided with the jaws of that chuck. Apparently somewhere in it's commercial life it was just a bit too small for some critical operation that needed to be performed. Rather than purchase a new machine or farm out the work the company modified it to fit the job.
As mentioned in a previous post I don't like to stress the machines to the point they may fail. With that in mind I always try to purchase something that will put the maximum size of the work I do around 75% of the machine's capacity. In my opinion it not only extends the life of the machine, it literally minimizes the overall cost of the machine.
As for lack of tooling I think parts for older commercial machines are more readily available at less cost than those available for hobby class machines. These machines are now a small portion of the commercial market. They have given way to faster, more capable CNC machines in almost all production shops. It may be a function of where I live, but I have never paid more than $100.00 for a used chuck for any of my machines. In total there are more than a dozen, ranging in size from 5" to 10". There are about an equal number for each machine in both the 3 and 4 jaw configurations. Last year I purchased a like new 10" Skinner 4 jaw chuck with an L00 backplate for the Sheldon for $100.00 from a local used equipment dealer. By comparison I paid nearly $200.00 for a new 3.1" 4 jaw chuck for model work.
If I were in the market given what you've found my preference for machines in order would be 1 Cincinnati, 2 LeBlond, and finally Harrison. To be truthful I probably wouldn't consider the Harrison, mainly because of it's size and lack of rigidity compared to the others. However the type of work you do may be different from what I do, and may dictate a different choice in machines.
In any case I would courteously negotiate a price with the owner of which ever machine you pick. I have found over the years that even if you can't come to an agreeable price on a current machine the fact that you proceed through the negotiations in a courteous manner may help with future purchases. This is especially true if you are dealing with professionals in the industry. I leave a business card with anyone I've attempted to buy machinery from. More than once that information has been passed on to others in the industry that are upgrading, downsizing, or changing the type of work they do. It has resulted in calls and offers for machines I have been looking for. In several cases the offers have been too good to refuse.