thank you.
better ask the seller for the recovery dvd and make sure it's 10.6.*? i found a 2011 imac online and the seller says it was shipped with 10.7
All 2011 (early/mid/late) iMacs, MacBook Pros, MacBook Airs, and Mac minis which shipped with early builds of Lion are completely capable of running 10.6.8 natively. All are Sandy Bridge models.
Many, especially from July 2011 onward, may have been bundled with 10.7 or even 10.7.1 or 10.7.2, but these will run 10.6.8 — whether you install from a 10.6.7 DVD; install from another Mac on an external drive, followed by moving that drive to the internal of the destination 2011 Mac; or transplanting a build of 10.6.8 on an HDD/SSD from another 2011 (or even 2009) Mac, into the 2011 Mac you’re planning to run SL.
This is what I did when I moved from a mid-2009 MBP running 10.6.8 to a early 2011 MBP which shipped with Lion 10.7(.0): I yanked out that HDD and dropped in the HDD from the 2009 MBP. (Later, I cloned that HDD to an SSD and dropped that in.) Years on, I still run that SSD with the same build of 10.6.8, except it’s now in a late 2011 MBP — a model which (probably) shipped initially with 10.7.2.
Where you will run into trouble are with the Ivy Bridge Macs, which began being sold in 2012. While there are hacks to get SL 10.6.8 to run on them, they will not run without a hitch.
So in short: a mid-2011 iMac which shipped with 10.7(.0) will boot 10.6.8. You will just need to supply that yourself, whether by transplant and/or a 10.6.7 DVD.
As for best plan, given limited resources: the 2011s are pretty inexpensive now, and in some cases might be given away for free on local trading boards (since people just want the “stuff” no longer in use out of their lives). Heck, I’ve even run across Haswell-era 2013 iMacs being given away for free locally.