Thoughts on Where to Start with 3 Computer Options

The T7500 is an older machine from 2009, so it's tired and past it's prime. They're not bad machines for the right job, but the age makes it a hard sell. For buyers who know better, the T7500 is the worst of all worlds; age, no Xeon support, Core 2 Duo/Quad only and power supplies are normal but if they fail then you will have a case gap so it's either finding a used one or living with a gap. If I, were you, take the first reasonable offer and let it go “as-is” to avoid being on the receiving end of a refund unless you can do a multi-day “burn test” where you leave it on all the time to make sure the odds of bad caps showing aren’t too high. Personally, because of the age of the system, if someone offered be $50 for it, I’d take the money and run far away. When it comes to those power supplies, it isn't unheard of for the older Dell BTX power supplies to have capacitor issues, even after 2005-2007 motherboards began to have fewer problems; it's often bad capacitors inside. I would get a good one like a Seasonic, or sell the computer for parts and wash your hands of it.

The T3500 is in the same group as the T7500 in that time has kicked it to the curb due to age of the CPU and chipset. While they support LGA771 Xeon processors (which makes them a bit more desirable), don't expect a lot for it unless you luck out. Throw an older GPU at it like a passable Fermi that works, see if it takes Win10 nicely and if it does sell it as a budget gaming PC but be honest; it’s not a modern i7 powerhouse. Take the most reasonable offer you get, and be happy with it.

If possible, keep the T3600. Those aren’t burdened by volatile capacitors known to randomly die because you hurt their feelings, but the power supply is likely nonstandard due to the case design (that said, it’s not like the “built to a price” 250-300W Inspiron DC supply). I’d also check the CPU installed against the CPU list for Win11. Most old Xeons made the cut (which makes the “security” excuse more suspect, as Xeons based on similar silicon were added but not Core i variants?), but it’s worth doing a sanity check since the list is so particular.