The 3 and 4 are near identical, apparently, I'd just get the ipad 3. The big difference was between the 2 and 3.
The 3 only came out 4-6 months ago so hardly old or out of date tech.
the 3 is now deleted. You can only get the 2 or the new one (4). They replaced the 3 with the 4 when the mini came out. I think you can still get the 3 in the refurbished section of the Apple store, but I'm not sure if that is US only.
Is there a reason you are discounting the mini. we have gotten a mini for my DD for christmas and I just took it out of the box to check it last night and it is great, so light. I have the ipad 2 and I love it but it is quite heavy to lug around. I can't imagine having to put 2 in my bag if we were out whereas the mini seems nice and light.
The 4 and the mini (and also the new ipod touch) have different connection points, so the bit that goes into the power point is the same, still a USB attachement into that but the other end is smaller. This also means these new ones won't sit in any docking stations/speakers etc that you already have.