Choosing a compatible laptop: Ubuntu Certified Hardware page complicated, help needed

Hi
I have to buy a replacement laptop and have been looking at the Ubuntu Certified Hardware page.

My priorities are:
I don't mind if Ubuntu is pre-installed but whether it is or not I want to be able to load different linux OS from scratch, on separate partitions, so I can compare them. I have some experience in installing dual boot systems.
I want SSD drive. Minimum 128GB. I could install myself but prefer not to.
14” screen
Budget £500

I have narrowed my search down to
Dell Latitude 3470 cost £390: Dell wireless card, Intel graphics, 128 SSD
Lenovo Thinkpad 13 cost £530: Intel wireless and graphics. (I see some Lenovo models have a locked SSD)
Both are on the certified hardware website as pre-installed, but with many different components from those listed on the manufacturer's site in the specs.

My problems with the Certified site are these:

“Pre-installed” has a warning that standard images may not work at all. What does this mean? It would seem to imply that it's not safe to install any updates.
What does “Available from Ubuntu.com.”mean? Does it simply meant that you buy the model and install yourself?

Most examples on the site have up to 50 hardware items specified. Is it now up to me to ensure that if I buy a listed model it has exactly the same hardware components or it might not work with Ubuntu?

Is it therefore safer to buy “Available from Ubuntu” options?

Is it possible to find out which individual components are supported by Ubuntu 14:04 or 16:04?

Finally, is there certain hardware that is much more likely to be supported by Ubuntu? For example is it a safer bet to go with Intel graphics, cpu and network/wireless hardware? What about sound cards and SSD?

I was put off Hewlett Packard due to reports of no wireless drivers provided beyond 12:04. But if HP don't supply them is it possible to get them from the wireless card manufacturer?

I want to play very safe on this and don't mind restricting my choice or even paying extra.