Well, after considering the various options and my own space restrictions, I've joined the Quickjack club. Its relative flexibility and portability and the fact that it leaves an unobstructed workspace under the car swung the decision.
After Stephen's warning on the weight I was rather taken aback when the delivery driver - half my size - nonchalantly slung the jack package over his shoulder and asked where I wanted it! The lifts are (perhaps reassuringly) very heavy - I struggled to carry them the short distance to the garage! ( I guess he was half my age too). The wheels on one end do help when moving them about and after fitting everything together and filling the pump with ISO 32 hydraulic oil, positioning them under the car is relatively simple.
I too went for the 5000 lbf model. As has been pointed out, the C3 is just about on the limit of the 3500 lbf unit and moreover I need to use it under six different cars, the heaviest of which is my 'Stang at 500 lbf more than the 'Vette.
It was the 'Stang I used for the first test since its jacking points are very flexible and I figured if the jack raised this, anything else would be no problem.
Must say I was impressed with the ease it went up and once locked car felt very stable. The current Costco offer was the cheapest I found - it came in at just over £1k. This is supplied with the 12V power unit which I thought was fine since I could power it with my Smart Box jump starter......

For some reason this didn't want to play ball with the Quickjack - I'm not sure why - so I had to use a spare battery.
Partly because of the weight but also useful with limited garage space, it's very convenient being able to store the jacks under the car when not in use.
Thanks to those who contributed to this thread, your input helped my decision. All in all, car maintenance in my twilight years just got a little easier.