IOS development on Mac mini mid 2011 I'm about to dive into iOs programming, which is a whole new area for me.
One of my friend said he's gonna sell his mac mini mid 2011 device, which is a little hardware upgraded.

My question is, can I develop applications  targeting most recent IOS version with this machine. Is there a restriction like, this mid 2011 machine's OS version can only have XCode version up to something, and up to something versioned XCode can only support developing IOS applications targeting not most recent version of IOS? 

Or shortly, can I develop iOs applications that can run on most recent IOS version with this machine?
In general, if your Mac runs the current OS - you can run all the Dev tools needed. In specific, that Mac meets all requirements for OS X 10.11 and Xcode 7
Xcode 7.3 system requirements
You want to start your machine to support Xcode 7.3 (which is now still in beta, 25 feb 2016). Xcode 7.3 beta requires a Mac running OS X 10.11 or later.
These Mac models are compatible with OS X 10.11, aka 'El Capitan':

MacBook (Early 2015)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
iMac (Mid 2007 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)

So, you should be good to go and start developing!
Source:

https://en.wikipedia.org/wiki/Xcode#7.x_series
http://adcdownload.apple.com/Developer_Tools/Xcode_7.3_beta_4/Xcode_7.3_beta_4_Release_Notes.pdf
https://support.apple.com/kb/SP728?locale=en_US