..Recently, when I compare T3620 with T5810, with similar CUP and Graphics card, T5810 is more expensive than T3620, could you explain why?
I helped a family member who used a Dell Dimension for their business. They used it everyday and depended on it. I spent many hours improving it, quadrupling the RAM, tweaking their Windows, changing out the hard drive for a larger faster one, oiling the single fan inside, backing it up, etc... It went from a very slow desktop to being noticeably faster and quicker.Rambling thoughts...
Besides the money saved purchasing used, you can get high quality and reliability.
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My system runs 4 workstations. I've been running Dell Precision (Dell's best quality line, IMV) for at least the last 10 years. And before that, I ran Dell Dimensions which were also good and reliable.
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10 years... 4 computers... 100,000 hours, only 6 minor repairs. THAT'S why traders should at least consider work station-class computers for trading.
FWIW...

Checkout craiglist and ebay for commercial workstations off lease. Purchase top-of-line workstations, desktop and laptop for about .10 on the dollar.

I helped a family member who used a Dell Dimension for their business. They used it everyday and depended on it. I spent many hours improving it, quadrupling the RAM, tweaking their Windows, changing out the hard drive for a larger faster one, oiling the single fan inside, backing it up, etc... It went from a very slow desktop to being noticeably faster and quicker.
ALL of the 12 larger/taller Dell Dimension's motherboard capacitors leaked or bloated upwards. The system started freezing up in Windows. It wasn't worth the effort to replace the capacitors so I helped them buy another computer. They ended up giving the Dell Dimension away to a small store which takes any donations. I wrote a note telling the store about the problem and how a young person could replace the capacitors and have a working computer. They probably threw the Dell away.
Here is a picture of some of the Dell Dimension's leaking capacitors. The leaking capacitors are the tall black and light blue ones. ALL 12 OF THEM LEAKED OR BLOATED UPWARDS. The capacitor on the far right bloated upwards. This is why I don't buy prebuilt computers, especially used ones. You never know what you're getting.
I have been building my own computers for many years. I usually buy the highest performing components available. This way I don't have to build a new PC for a long time. Building, even only buying, a new computer is very time consuming. In addition, you must install your own software, tweak it, tweak Windows.
This is my current computer. See only the first and last minute. I built it, tweaked Windows, installed all the software, etc...
Spec says no wifi and bluetooth. Space permitting, a refurb professional HP or Dell with intel quad core xeon workstation for multiple trading platform and office apps, goes for about the same price or less. Imagine with weight of all the cables would be more than the unit. But if it works for you, go for it. Send a picture of it fully operational with 4 monitors running a few apps.Just get a Lenovo Thincentre M9x Tiny for less than $200 from Ebay.
Some comes with 3 years onsite warranty.
This little devil can drive a 4k monitor or a total of 5 Monitors 1080p.
and spend the $ on a monitor that will be gentle on your cornea.
Spec says no wifi and bluetooth. Space permitting, a refurb professional HP or Dell with intel quad core xeon workstation for multiple trading platform and office apps, goes for about the same price or less. Imagine with weight of all the cables would be more than the unit. But if it works for you, go for it. Send a picture of it fully operational with 4 monitors running a few apps.