Planning on using a graphic card to transcode 4k 2 or 3 videos per time. Just want something out of the box that doesn’t have to be configured.
This is contradictory. If you want to use a graphics card for transcoding, you must first configure it. As a starting point, you may want to review the documentation:
https://jellyfin.org/docs/general/administration/hardware-acceleration/
WHAT KIND of router? Can you save configuration? Can you reuse that configuration on another device?
Can anybody recommend a cheap computer that can run Linux and has an Ethernet port with built in LTE or 3G?
First off, 3G is either dead or dying. All major U.S. carriers, for example, shut down 3G service in 2022. Globally, it's probably similar. So your minimum is LTE.
To answer your question, no. Unless you happen upon a used device and the seller doesn't know what they have, this is not cheap tech. And when it is cheap, it is often because it has vendor locks.
For example, I once bought a Datto DNA-VZ5 (a rebranded Axiomtek NA361) from a recycler. It had six Ethernet ports (4 x Intel i354 + 2 x Intel i210), Wi-Fi (Qualcomm Atheros QCA986x/988x, which is AC standard), and a cellular modem (Sequans VZ20M). The problem is, there are no open-source drivers for Sequans VZ20M. Further, the U.S. version is locked onto Verizon, and the Canadian version, onto Rogers. Further still, the device is somewhat larger than a typical desktop router, actively cooled, painted bright blue, and has the total of five antennas (three for Wi-Fi and two for cellular) on two opposing sides of the case. So even if you were able to make it work (for example, by replacing the Sequans card with a more open-source-friendly one), it still wouldn't meet your aesthetic criteria.
Here's what you can do. Find out what devices your cellular provider offers, then see if you can find those or similar in the secondary market... Most likely, you will end up with a standalone cellular modem. You may be able to find a cellular modem implemented as a PCI card (which you can install into any PC with a PCI slot; those are usually SFF or larger) or as an m.2 / mPCIe card (which you can use in a mini-PC if you take out the Wi-Fi card and replace Wi-Fi antennas with cellular ones). But either way, you're highly unlikely to keep it under USD 100...
Nay, emphatically. Out of the box, a new SMTP server is treated by peers as yet another spam delivery vehicle. You have to prove to the world you're not a fly-by-night spammer. There are certain things you have to do with your domain's MX record, as well as in terms of SMTP server configuration. Oh, and if you're ever caught with an open relay on port 25, that will get you blacklisted instantaneously...
Any thoughts on the reliability of the business (OptiPlex) versus consumer (Inspiron) desktop
None whatsoever. From where I sit, it's all about you accidentally spilling liquids into the unit. Or putting the unit somewhere where said liquids can condense from the air and short out the motherboard...
Also, some time in the 2010s, Dell adopted model numbering in which 3xxx models are "value", 5xxx models are "mid-range", and 7xxx models are "top of the line". There's also the 9xxx range, but that's usually either Precision or XPS. That, not the product line, is the real clue. If a 7xxx model has a motherboard with four RAM slots and four PCI slots, a 3xxx model of the same vintage may have two of each and ship without an HDD cage (with only an SSD on the motherboard). That sort of thing...
Go to Dell's Web site, find service manuals for both models and see which fits your plans better. Down to the number of available SATA ports and the specs of the PCI slots.
Also, Dell's top desktop line for business is Precision. There's also XPS, but these days, it's mostly Precision in a slightly fancier case... Precision lineup goes all the way up to Xeon...
I bought a QNAP device a decade ago. Today, it is about 3% full. Turns out I didn't need any NAS at all...
I look after two AdGuard Home installations.
One is local, running on a super-tiny PC (Intel Atom x5, 4 GB RAM, 64 GB eMMC, Debian 12, and I see no reason why AGH wouldn't run just as well on a 2 / 32 GB version of that PC). The average handling time for a DNS request is 30 ms. You could easily do something similar in a Proxmox container, give it a local IP address, and have you router use it as the DNS server instead of whatever it's using now.
The other is in the cloud, running on a virtual server with 1 GB RAM. The average handling time for a DNS request is 10 ms.
It's really hard to tell, and the devil is in the details.
On bare metal, a single server containing both the front-end application and the DB may be faster (no networking overhead), but only to a point. As load increases, a split system (front-end on one machine, DB on another, or even on a cluster) becomes more attractive.
When everything is virtualized and machines talk to each other over virtual interfaces, I would think a single database server usable by multiple front-end applications would be a good idea. This way, you have only one DB server overhead. Also, maintenance is more straightforward; you look after a single database server, even though it contains multiple databases.
It's probably a good idea to pair each application with a dedicated database (within the same database server) and assign each application a unique user name with rights only for that database.