Having been avid cyclist, all my life i would recommend that you seek out a 'real' cycle shop in your area; and solicit their advice on what to buy; type, fit, price, etc.
By 'real' i mean a member of the renewable energy 'mindset'; not a member of the 'capitalist pigs' sales confederate.
Check your local news for bike friendly groups like the one i belong to:
CBN (Toronto)
and let them guide you to an environmentally friendly; and knowledgeable bike shop.
[and as far as the seat goes, for example, the worst ones are the softest ones; good ones take a little 'wearing in', but in the long run support you much better, and are more comfortable - but you need to be told that in advance!]