With respect, do you not see how you are making talented engineers/technicans and CCTV companies, sound like chimps that plug things together. You wonder why you have ruffled a few feathers.
There is a lot more to installing a CCTV system then buying consumer products (DVD players) that are designed to be foolproof on installation.
In a professional installation every system is bespoke, designed to get exactly what the customer needs. There are so many variables and choosing the wrong parts could lead to a seemingly working system. However, the evidence maybe not fit for purpose.
The community is very helpful, but they can't teach you there whole job overnight (this could be 3-5 years apprenticeship and xyz years experience).
How longs a piece of string, this is the important part.
You need a good quality camera to get good images.
If you need identification we are talking megapixels, however, we don't know the viewing angle, field of view, distance of it's location.
They are sort of the same thing. DVR is normally used to describe analogue systems and NVR for IP.
Both normally have Ethernet connections.
Both store on to hard drive and you can control the recording rate to last xyz days.
The larger the HDD the more you can store before overwriting it.
Cat 5e and Cat 6 are standards of network cable.
https://en.wikipedia.org/wiki/Ethernet_cable
May not be required if using a PoE switch, see below
Not necessarily needed. Depends on the site.
To connect up an IP system you will need some IT networking skills.
https://en.wikipedia.org/wiki/Network_switch