I would say minimum requirement would be FSX deluxe & REX, priced @ as stated above, plus a reasonable joystick to get a bit more control over the actual flying. As for scenery/planes & mission etc, there is plenty of freeware to keep you busy for years and tons of payware if you need anything specific which you feel cannot find in freeware.
There is simply so much you can do with FSX, mostly to do with flying of course, but you can even drive cars or ships about. The FS community is what is keeping FS alive, not just the tons of freeware/payware addons but also online flying, learning about flying, enjoying the view around different parts of the world or admiring different aircraft for their modelling or perfomance, airtraffic control, navigation, doing missions or simply fly any type of aircraft you ever fancied having a go at. Whether you're in it for wanting to learn for your PPL(private pilot license)/retired or wannabe pilot who enjoys keeping their flying skills up or just a casual flyer, FSX has it all.
But it may not be everyone's cup of tea, don't expect awesome visual & performance at the start unless you have a high spec PC and decent addons to FSX (whether freeware or payware), don't expect intense flying action like in a combat sim (HAWK for instance). So knowing that you do need to invest @£80-£100 for both software and a decent joystick, I would carefully consider it first and do plenty of reading and research into the Flight Sim world & community before taking that leap, but if the bug does grab you then believe me, no other PC 'game' has given me more hours of use and joy then FSX has, not even a few MMORPGs which sucked a few years of my life and left me feeling I wasted them. Whereas with FSX, although I properly never learn to fly, I do have a satisfying feeling that I learned something about flying and I can see myself doing this more and more especially the closer I get to retirement.
My 2c's.
It's good enough to get me on the ground and with a few of my approach controls (flaps etc) on it it does the trick (use kb mainly while in the air). It's definately lacking in finesse though.
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a bit more than I thought, but it looks v.nice with those two add-ons.