Amazon has a great buy on an Acer Predator desktop computer for about $700.00 including shipping:
Acer Predator AG3-605-UR1C Desktop (3.1 GHz Intel Core i5-4440 Processor, 8GB DDR3, 1TB HDD, NVIDIA GeForce GTX 745, Windows 8.1) Black
This is the base computer that I use, with a more modern Intel processor (I-5) and different graphics card, the NVIDIA GTX745. You can easily upgrade to a more expensive and powerful NVDIA card (the 760) if you need to, but this one is very good- it should allow you to run Second Life in high with minimal lag, and ultra for stationary photography (depending on your connection of course).
I recommend you only consider Intel and NVIDIA combinations for Second Life. They run the best.
This runs Windows 8.1. At some point next year, Microsoft should make it easy for 8.1 users to upgrade to Windows 10, which should be out next year and is getting good reviews. I would not worry about the operating system right now.
The Predator is well made and quite attractive with cool red lights.
Make sure to take a warranty if you buy this. Cheap ones are offered on Amazon, but I prefer to use Square Trade since they are so good at paying out if necessary.
Here are details:
4th Generation Intel® Core™ i5-4440 processor 3.1 GHz with Turbo Boost Technology up to 3.3 GHz (6MB Total Cache)
8GB DDR3 Memory
1TB 7200RPM SATA3 Hard Drive
NVIDIA® GeForce® GTX745 Graphics with 4GB of Discrete Video Memory
Digital Media Card Reader -Secure Digital™ (SD) Card
10/100/1000 Gigabit Ethernet LAN
802.11a/b/g/n Wireless LAN
8X DVD-SuperMulti Double-Layer Drive
High-Definition 5.1-Channel Audio Support
4- USB 2.0 Ports (2 front, 2rear)
4- USB 3.0 Port (2 front, 2 rear)
1- HDMI™ Port
1- DVI Port
USB Keyboard and Optical Mouse