As someone with an masters' degree in ergonomics and some career spent in office furniture and allied trades, my recommendation is absolutely NOT to buy without trying. Work chairs are like shoes - one size most certainly does not fit all.
Search online to see if there are any dealers in second-hand office furniture around your area then get yourself down there to see how the chair works for you and how adjustable it is. Points to check are:
* does the lift/lower mechanism work smoothly? If not, it's probably on its last legs and best avoided.
* does the chair have a tilt mechanism and, if so, does it work smoothly? Tilt mechanisms are really handy - I tilt forward to use the machine and back to hand-sew
* does the chair have arm rests and, if so, are they height-adjustable? Arm rests are not vital but if they are present they should not inhibit you getting as close as you need to to your work surface.
* does the seat back height mechanism work and can you get it to a point where your lower back is comfortably supported?
* what floor surface are you intending to use it on? Castors/glides come in different forms for different surfaces and chairs intended for hard surfaces are grippier than those intended for carpets, which can make the chair awkward to shuffle around on if you have pile carpet. Carpet castors/glides can be a bit dangerous on hard surfaces as the chair has a tendancy to fly away from you.
* is the foam sticking out/worn & torn? If so, probably best to avoid - it's too far gone because office chair foam tends to be injection moulded onto the seat pan so can't easily be replaced. Unless the chair is otherwise a winner and you're happy with the challenge of course!
If the upholstery is a bit worn, it may not matter provided that you can see how it might be recovered/replaced.