Rev 11/26/18
KDE Neon is one of the most difficult types of Linux to use without a direct internet connection, due to several factors, although they can be overcome by using a temporary direct connection for initially setting it up, such as by updating the package index and adding Apt-offline, kdesudo, Synaptic Package Manager, and Software & Updates (software-properties-kde for the KDE desktop, and software-properties-gtk for everything else). (The software manager provided with Neon is clearly intended for use with a direct high-speed internet connection, and not for customizing the package index to your needs.)
Ubuntu itself was also intended for use with a direct high-speed internet connection, although it doesn't require as much initial set-up as Neon to make it suitable to use as an internet-access-impaired installation. (Just update the package index and install Apt-offline-gui and any other apps you want initially.)
If you can afford a laptop PC which can boot Linux from a USB port, setting-up a flash-drive installation would be easy, and the installation could then be used for booting another PC, including a desktop which could be used even for video editing (the video files would be stored on an hdd/ssd, perhaps in unencrypted form due to the amount of processing required to encrypt video). However, such laptops tend to be expensive, and I don't like laptops in general for various reasons. There was a time when you could get a small laptop ("netbook") without an OS (and without built-in wireless!), into which you could plug "Linux on a stick" and boot it, but this didn't sit well with Big Bro, who didn't want it to be convenient for everyone to get a PC which he couldn't monitor. (Based on Snowden's claims, the Thought Police are a secret division of the NSA.)
So, netbooks disappeared from the "free market," and if you want something with the same capabilities without bowing down to the laptop-dictatorship, the best alternative is to use an AMD-based mini-PC without built-in wireless, and with a wired-only keyboard and a wired-only monitor, because it could double as a secure home-PC. (At home, I have cheap "offline" and "online' mini-PC's which run Linux and share the same keyboard and monitor through a KVM switch. I don't have to worry about hackers, viruses, or updates, although I replace the OS on each PC every year or so to stay reasonably current.) Portability is compromised, because you'd probably need an AC outlet. Perhaps you know of an access-point where you would be allowed to use AC outlets, but if not, you could get a DC-to-AC inverter and run it from your car's electrical system. There are small monitors which are designed to use in cars, or you could use a regular monitor when there is little sunlight, or somehow block some of the sunlight (perhaps put a cardboard hood on the monitor - typically on the top and sides, and about a foot deep, although it could be something like a box with a slit in it). You could use drive-in restaurants as your wi-fi access points (you'd use an external wi-fi adapter, some of which plug into Ethernet ports and don't require special drivers). These are just some possibilities, and you can probably come up with some others.
But once the installation is set up, it would be much more convenient to use Apt-offline to make changes, and if you want to make some changes to a secure installation, Apt-offline allows the changes to be made without compromising security.
But then there's always Xubuntu, which includes Apt-offline's text-command version by default (which makes it easy to install Apt-offline's GUI, which comes in handy at times). The decision to include Apt-offline's text-command version is a reflection of Xubuntu's intelligent, minimalistic overall design. It's a little rough around the edges, but Ubuntu and its derivatives are undergoing major changes (such as the development and incorporation of a new display-server known as Mir), and rough edges are to be expected until the dust settles.
Revision notices
11/24/18 - A) Corrected the 1st sentence in the 4th paragraph. In the initial version, I accidentally wrote "wireless" when I meant "wireless-less," i.e. wired-only. If wireless capability exists, you should assume that it can be surreptitiously enabled, such as in burst-mode, to spy on you. Wi-fi burst-mode is a reality, and it cannot be detected without special equipment. So, a secure PC cannot have any wireless circuitry, period, including in any peripherals. The NSA's TEMPEST-spec takes this to an extreme - it considers even wire to be a wireless device. B) Clarified the last paragraph.
11/25/18 - A) Revised the sentence beginning with "If you can afford a laptop PC..." for clarity and to mention the use of an hdd/ssd to store video files.
11/26/18 - A) Tweaked the sentence beginning with "If you can afford a laptop PC which...." B) In the sentence beginning with "There are small monitors which are designed to use in cars," changed "shroud" to the more common term "hood," and suggested a couple of basic designs for the hood.
