In reply to rallymania:
I'm in the same-ish boat - Have a 35mm and 85mm Sony lenses, and some vintage ones too, but want something longer.
Sony lenses are pretty pricey, and only rarely come-up second hand (though if you're on the Talk Photography forums, you occasionally get lucky).
I hired the 70-200 F/4 for a trip to Canada and lover it. By repute, the F/2.8 isn't worth the extra money unless you really need the shallow DOF for portraiture.
Part of me is leaning towards a Canon lens and an adapter as they can be picked up much cheaper second hand.
The Sigma options aren't that much cheaper than Sony ones these days.
Sorry, not much help - but some comfort in knowing you're not alone?!