I'm targeting people with decent computers, something that can render 3D graphics with post processing at a playable fps....
In that case go for 3.x - it's all achievable with earlier versions for sure, but you'll have a much nicer time using 3.x.
One project I was involved in up to maybe this time last year (where initially I had thought I was being brought in just to optimize the renderer), one of the leads was absolutely insistent on the "what about older hardware?" line but yet was also pushing very heavily for lots of post-processing, lots of complex geometry, lots of real-time dynamic lighting, etc. I ended up with an insane mixture of core GL1.4 with a software wrapper around VBOs, ARB assembly programs, glCopyTexSubImage2D, multiple codepaths for everything and an edifice so fragile that I was terrified of even bugfixing it (the fact that it was build on an originally GL1.1 codebase that was fairly crankily and inflexibly maintained to that point didn't help). It was a nightmare - I walked out one day without saying a word and just didn't come back.
It's just not worth going down that route - you'll only burn yourself out. So either dial back the ambitions and use an earlier version, or else keep the ambitions and use the most reasonable sane recent version. But don't try to mix the two.