Are you talking about for the average uses of a PC (eg. browsing, music/video/image storage/playback, office apps, browser gaming)? If so, an i7 is like a ferrari to pop down to the shops: It's total overkill. I have a spare first-gen i7 PC but I can't imagine ever selling it to my typical customers because it's huge, eats power and frankly many more modern processors do better at single-threaded tasks. An i5 sandy is probably a pretty good choice for someone with the average needs and who might branch out - if it's not too late by then for that cpu. But bear in mind in say 2 years time an i5 sandy is going to look like a first-gen i5 does now.
I would only buy a used mobo as an emergency measure. I don't buy cheap boards either, simply because they're the part that is a make-or-break failure for an aged PC: If it dies at say six years or older, it quickly becomes a question of "should I just be replacing the whole thing", because for all you know the board in question was later found to have a long-term reliability issue. Pretty much every other component in a PC that age is an obvious candidate for repairing the PC rather than replacing it, unless the customer's needs have moved on significantly since the PC was built.
I just replaced my wife's G620 with a 2550k I got off ebay. The way that she uses her PC now (ie. she uses her phone as her primary device now), I suspect that the only reason that we'll be replacing that PC is when it dies, as I also took the opportunity to upgrade it to Win10 with an SSD. If I had left it on a G620 even with Win10 and an SSD, that CPU is unquestionably going to show its age with a standard browsing workload in the next few years, if it doesn't already; all it needs is for Windows Update or say a background scan to consume a core, then the other one will be overloaded with browsing work.