Thanks to everyone who posted here. It was really useful for me.
I thought about this a bit further. I was going to add casters and then build the torsion box to bring the table up to the same height as my Triton workcentre. That is 900mm high.
I also don't want it too low due to my back pain.

Now I'm thinking perhaps I could live in hope that I might be able to buy a real table saw next year. That is probably wildly optimistic but who knows - it could happen! When I do get a table saw, it might not be exactly 900mm. Is there a standard height?

Also I realised that I have a Z-vice 2. The first time I used this a wing nut broke off. So in disgust, I tossed it into a cupboard in the garage and forgot about it. That was about 5 - 6 years ago. I don't have the manual but I have all the parts (minus this wing nut).
Zvice.jpg

Now I remember that I have it and now that I have a table, I could use it. That would give me some sort of vice until I make a workbench. Much better than no vice at all. This thing can clamp onto up to about 65mm. Not the 70mm+ I was going to end up with if I built a torsion box on top of the table top. I just need to figure out how to replace the clamp part with the broken wing nut. I emailed the Z-vice guy in NZ but I did not get a reply.

So now I'm thinking I should take the advice of some on here and just keep the table as simple as possible and cover it with something sacrificial. Later I can remove it and build a torsion box to the height of my table saw, when I get one. I decided that some hardboard was not thick enough to strengthen the table. I'd like it more solid. So I thought I'd get some MDF or ply and went off to Masters.

On Tues Masters had these sheets of form board going cheap as chips. I was not familiar with it before. It seems to be particle board laminated on both sides with a smooth surface. The young guy in the yard convinced me to go for this. Probably this board is a bad choice if I want dog holes and screws etc. because I'm not sure how particle board holds up in that respect. It is also very heavy which is good and bad. But hey it was going cheap and I'll only screw it on so I can remove it if and when it deteriorates. It is very rigid so it seems to be very flat. (I could check that with some winding sticks if I want to be accurate.) I got it cut slightly oversize in the store. I'll trim it to size before I screw it onto the table.

Will 19mm be enough depth for dog holes? I just realised I'd have to drill these first, if I want them.

I also got some FJ moulding to trim it with and for the underside for clamping onto. (50% off).

While I was there, I also bought the casters. I started to worry that my table will end up heavy if I keep adding things to it, so I went for 75mm casters. No doubt that is also overkill but the extra height will be a good thing for my back.

I have to finish some painting in the house before I can do much else. (I don't like painting but I've been promising the Mrs I'd do it for 3 years.)

Next I'll need to decide how much care I want to take to brace the legs. I'll do that before screwing the board to the top because the board makes it heavy. I could just bolt some framing timber to the legs or I could learn to make a mortise and tenon joint on a saw horse.
mmmmm.... think I'll just bolt something together.