Refurbished PCs on eBay

*7000-9000 series are the same
*i5 versions are cheaper and just fine. Both have 4 cores but i7 has 8 threads compared to 4.
*8 GB ram is minimum so factor in buying 4 GB ram if buying 4 GB PC.
*Adding a 256 GB minimum SSD boot drive is a must. Larger GB is faster. Smaller works fine but 256 is good compromise if loading programs to second drive. I'm shooting for 500 GB deals lately.

optiplex 7010 mt i7 (Core i7-3770)
optiplex 9010 mt i7 (Core i7-3770)
optiplex 7020 mt i7 (Core i7-4790)
optiplex 9020 mt i7 (Core i7-4790)

Boots M.2

Dell Optiplex 7040 MT i7-6700
Dell OptiPlex 7050 MT i7 7700

7040-9040 seems to be earliest support for M.2
These are going to be harder to find yet and more expensive.
optiplex 7040 mt i7 (Core i7-6700)
optiplex 9040 mt i7 (Core i7-6700)

Here is an example of what to look for.
See Below for setup suggestions.

Setup Suggestions:
Find the Product Key sticker for Windows 7 Pro . . .
Install Win7 Pro and Activate. You can now activate Win10 with Win7 keys
Download the Media Creation Tool

Some of these systems come with the NIC setting in BIOS causing a boot screen like this . . .

Simply re-boot (F2) and change this . . .

Memory Compatibility:

i7 3770 (Dell 7010-9010)

Refurbished Computer Links

Core 2 Duo is still great for business stuff.

It is best to enter the tag number into the Dell support site but here are a couple drivers that are hard to find. They may be different for some models.
Optiplex is missing these two drivers and Intel does not find them:
HECI
ftp.us.dell.com/.../Intel_AMT-HECI_A02_R255437.exe
SOL
ftp.us.dell.com/.../Intel_AMT-SOL--LMS_A02_R255438.exe

Just buy from dell if they have them. Sometimes others have them with more ram, no tax and free ship so then buy from that vendor.

Actually Dell has them for less but less ram, hd, etc.
http://www.dfsdirectsales.com/dell/ctl641/cp19257/cl2/optiplex

I just bought a Dell Optiplex 760 for $240 tax/ship. It has a core 2 duo 8400. Surprise, it had an 80 GB Velociraptor that reads close to 150 on HDTune. The full wide tower easily takes a second HD (bracket included and power cable ready. These are a great buy for business. Video card a Radeon 3450 and drivers were a snap with AMD Auto Detect. It came with Vista which really had to go.

http://www.ebay.com/bhp/dell-optiplex-745-intel-core-2-duo

Dell Optiplex line can be confusing when deciding but worth buying.

The sff/sdt line small form factor and small desk top look the same basically. Can not accept full height add-on cards. No cables or spot for second hard drive. Power supply is oddly shaped and not standard. PSU can be difficult to find and expensive. Buying a Core2Duo approaching 3GB performs great.

The usff (ultra small form factor) line is ultra small with external power supply which I found for $18. Otherwise same as sff/sdt. Performs well and very quiet. A cute little guy with punch.

The smt (small mini tower) is a tower. The models I received were huge and mini did not apply. Maybe they were full tower. All the same as above with the advantage of accepting full height add-on cards and standard cheap PSU. These are great for the office. easily throw in a second hard drive for backup. If you need two monitors just add a pci-e but make sure it is narrow because the pci-e slot is close to the heatsink plastic cover. That cover holds the heatsink down and you need it. You can buy Radeon 6450s for $15-20 after rebate. Make sure the video card does not need extra pci-e power or you will need to swap the power supply. It is also nice if low profile brackets are included in case you need to throw it in a small form factor case. I had only one of the video chips die but it can happen.