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...
With requirements like this, the answer is "anything". No, really. I have a Dell laptop from 2008 that runs Linux Mint on an Intel Core 2 Duo processor and has 8 GB RAM and a terabyte hard drive. It keeps up with my files without breaking a sweat.
In fact, you don't even need a 64-bit processor; you can run current version of Debian or Mint on a 32-bit one...
Exceptionally bad design; where's the cat supposed to sleep? :)
I bought a QNAP device a decade ago. Today, it is about 3% full. Turns out I didn't need any NAS at all...
It will be an exploration mission.
First, I can't remember whether XTM 505 is x86 or not; Watchguard has a habit of mixing designations. There are similarly named models with x86 and non-x86 (usually, Freescale) processors. Other people say it is, so I'll tentatively agree.
You most likely will not be able to run a Watchguard device with stock firmware. So you will need an alternative. The easiest one will be OPNsense nano; just replace the stock CF card with one containing OPNsense nano, and it will run. Same with OpenWrt; write it onto a CF card, and it will run. Or you can experiment with adding SSDs. This may of may not go well, because you may or may not be able to get into BIOS.
Performance-wise, XTM 505 will give you basic Gigabit routing no problem (except that one of the ports is actually 100 Mbps). How much noise it will make doing that will depend entirely on the state of the fans.
Peak power consumption on XTM 505, if memory serves, is 85W.
What you want to do with all of the above is entirely up to you...
The best place to start is usually eBay or a functional equivalent. Businesses dispose of the prior-generation tech more or less constantly.
Another poster suggested buying a server. While it may be a good idea from the standpoint of learning, I would like you to keep in mind that servers can be noisy, so adjust that recommendation to your living circumstances. An alternative is to buy a PC workstation or even a high(er)-end office PC (for example, Dell Optiplex, or HP Pro Desk / Elite Desk, or Lenovo ThinkCentre / ThinkStation). Those are designed for corporate use and have multiple options for upgrading and expansion (empty slots for RAM, SATA drives, PCI cards, etc.). You can start as low and as far back as i3-6xxx, but obviously, the more recent and more muscular, the better. See what you can get for your money in your home market. Get some extra RAM (16 GB should be enough for starters) and an SSD to install the OS on. Speaking of the OS, start by installing Proxmox (it's a Debian-based hypervisor, meaning, an OS designed to run other OSs on top of itself) and run the rest, whatever it ends up being, as virtual machines.
And yes, you can run your NAS as a virtual machine, too...
What you're looking for is called "JBOD enclosure" (JBOD stands for "just a bunch of disks")... Be sure to get a SATA one; there are also ones that work with SAS drives... Once you have an enclosure with some drives in it, you can connect it to a NAS server via USB or, hardware permitting, Thunderbolt.
These days, a lot of high-power computing is done on graphics cards. Some problems, including protein folding and finite-element simulations in engineering, are particularly well suited to being solved quantitatively on this sort of hardware.
Please elaborate on VPN. How fast do you want it and what kind is it? This will have meaningful implications for the choice of the processor.
Given your stated need for VPN, it would appear that Synology is out. Factory-built NAS devices are in most cases relatively weak in the processor department, so virtualization and computationally intensive things like VPN tend to bog them down.
Same logic applies to mini-PCs. VPNs can generate sustained high loads, for which passively cooled mini-PCs are often not prepared. So choose carefully if you decide to go that route.
But do you really have to virtualize your router?
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.
The big tower can stand on its own. As to various small pieces... Shelving. Regular open-backed shelving. IKEA, if you have one nearby.
So... Where does the cat sleep?
:)On top of the QNAPs? Is it warm enough?