Affordable GPU upgrade options for Optiplex 9020 MT without needing to upgrade PSU?

many better gpu options than gtx 1650 which sits at bottom of recommended gpu for Da Vinci.  https://www.richardlackey.com/best-gpu-davinci-resolve-nvidia-amd/
1650 meets minimum requirement 

• 4GB - Minimum for comfortable 4K work
• 6GB - You can tackle pretty much everything a 4K project is likely to require
• 8GB+ - You can work comfortably on projects above 4K resolution

a 250-280w psu is minimum you need to run i7-4790+1650+32gb ram + hdd.  running ssd only without dvd can save some power.  9020mt stock psu 290 W meets this requirement.

most affordable + trustworthy is new third party name brand 24 pin ATX psu from EVGA etc w warranty, plus a $15 or less 24 to Dell 8 pin adapter.  You can get a new EVGA 450w psu for $35. The whole setup cost $50.

or some user prefer Dell oem psu upgrade without adapter. In that case consider Dell XE2 MT 365w psu which is c/w 9020 mt.  T1M43 or 7VK45; Model: D365EM-00. new T1M43 $50 on eBay now.

that is a difficult question as it depends on seller.  in general if buy used better from an original owner you can trust such as a friend who can tell you how long they have owned and how much run time, whether it has been overclocked and gpu temp history. None of such info would be available from a refurbisher as they just test the gpu to verify it still works today but cannot estimate remaining life span.  A gpu well taken care of is supposedly going to work longer than one not, but each card life span also depends on model, maker, component quality.  In summary it is hard to predict.  If one cares a lot about investment then prudent to buy a new one of a highly rated model that has a few years warranty.  Cosmetic condition is one aspect of card condition but does not correlate well with card performance life span.  When someone buys a $60 RX570 used on eBay the expectation is that it will work for some time (?months to a year or longer) and not dead on arrival but no one knows how long it would last.

I have 32 GB of RAM, two internal 6 TB hard drives and an internal 2 TB SSD, as well as an i7-4790 CPU. Does this mean that there is already too much power being drawn from my stock PSU to allow adding even a GTX 1650 without overloading my PSU?

Also keep in mind that I'm not looking for the best GPU possible; I'm looking for the best one possible within my price range (which is $200 or less, ideally $150 or less). The only other models besides the GTX 1650 on the list that you linked that meet that requirement are the AMD Radeon RX 570 and several of the GTX 1660 models. The AMD Radeon RX 570 is actually the most affordable GPU on the list (it's currently less than $100 on Amazon in Used - Very Good condition), but pulls 150 watts, so I would definitely need a PSU upgrade.

Are there any affordable (under $100, ideally less) but trustworthy (I don't want to save money in the short run only to fry my motherboard in the long run) PSUs that I could upgrade to? The ones that I've found tend to be too expensive ($150 or more).

That does seem quite affordable for a PSU.

Regarding GPUs, is it generally safe to buy used and/or refurbished?

Also, how good do you think this option is compared to the GTX 1650? It's the AMD Radeon RX 570 model that I mentioned was the most affordable on the list (in Used- Very Good condition).

https://www.amazon.com/XFX-Radeon-Graphics-Black-RX-570P427D6/dp/B06ZXY43VC/ 

While searching Amazon, I came across a GPU that is currently very affordable new and has 8 GB of RAM. It apparently draws 185 watts. Could this be a good option, and would  450 watt PSU be sufficient for it on my system? It's a version of a Radeon RX 580, although some reviews say that it's a downscaled one.

https://www.amazon.com/AISURIX-Graphic-Express-Desktop-Computer/dp/B0BLCRNCX9/  

right off the bat I see this is a no name brand gpu which I would not recommend.  furthermore it tries to trick unwary buyer to think it is an ASUS card if one is glancing at the brand name too quickly.

450w psu is good for RX580 in 9020 Load Wattage: 387 W
Recommended PSU Wattage: 437 W

9020 have no more than 10 inches (25.5 cm) of free space for dedicated graphics cards. To be on the safe side the maximum GPU length you can use on OptiPlex 9020MT is between 9.5 and 10.0 inches. Everything above that calls for drive bay removal.

The size requirements rule out the AMD Radeon RX 570 4GB that I had linked (it's 11 inches long).

By the way, with the system that I have (two hard drives and an SSD, etc) would I need to upgrade my PSU if I would get a 75 watt GTX 1650, or only if I got a GPU that pulls more than 75 watts?

And if I wouldn't need to upgrade my PSU for a GTX 1650, would this one likely be a good option?

https://www.amazon.com/Gigabyte-GeForce-Graphics-Compact-GV-N1656OC-4GD/dp/B08BNRR5SJ/ 

some AIB card such as this GA gtx1650 requires a 6 pin PCIe connector that your stock psu does not have.  Nvidia reference 1650 does not require 6 pin but some AIB card added features such as OC etc. which needs the 6 pin.  you need to upgrade psu in order to use such card.

i7-4790 + 2 hdd + 1ssd + dvd + gtx 1650

stock 290w psu just meets load wattage

Load Wattage:289 W
Recommended PSU Wattage:339 W

"would I need to upgrade my PSU if I would get a 75 watt GTX 1650, or only if I got a GPU that pulls more than 75 watts?"  Only if more than 75w.  No need to disconnect DVD.

I'm with redxps on GPU - buy new and buy recognizable name brand.

Which drive is your boot drive, is it the SSD?  You could go internal with your SSD and external with an HDD used for storage.  Sabrent docking stations on Amazon  Single enclosures come in both fan and fanless models.

The SSD is my boot drive. I have two internal hard drives, but also have six external USB enclosures containing SATA drives.

I've started to think that it would be best for me to not bother with upgrading to any degree that would either risk overloading my PSU or require replacing it, though. The GeForce GT 1030 seems like a good option. It's very low in price and should give me a roughly a 2x performance boost compared to what my current graphics can do. It wouldn't likely be sufficient for the most recent versions of Davinci Resolve, but I've gotten used to using an older version anyway.

GT 1030 vs. RX 6400  RX 6400 = 50w.  If you do decide on the GT 1030, make sure it's the GDDR5.