Where to start?
It can be cheaper to buy components and assemble, but it will take hours, and if it is your first time, you will break things. Snapping together a chain or laying pulleys is really finicky work. You also need the odd bit of special equipment, to mount a cassette to a rear wheel, to put pedals on, etc.
Generally, I would recommend to get a well balanced bike, and then try to do some of the work on it yourself. For a lot of work I still head to the shop though, as they just have some equipment which it's not worth for me to buy.
Now, as for your budget...
Do you already have:
Bike shoes
cleats and pedals (oftentimes not included with the bike)
bib cycling shorts and a cycling top with bags for the replacement tube, pump, and tire hooks
a helmet
cycling glasses
stationary pump
hand pump
replacement tubes
cycling gloves
As you claim to know nothing of bikes, I surmise that this is not the case. This kit alone will set you back some 400 dollar. And most of it really isn't optional.
A comfortable saddle is all about how well it fits you. And how well the chamois in the shorts spreads the load. Generally it should only hurt for the first four months or so. After that, if you work on your stature, you usually don't feel the saddle anymore. At least not in the same way as in the beginning.
I would stay away from cheap carbon bikes. Why?
Usually the use extremely cheap components (7x Sora cassette? seriously?) and the carbon doesn't save weight, it actually increases it, due to the heavy gear they put on to balance the added cost.
With your budget, I'd check the local riders forum for a used bike, two year old aluminium frame with a 105 set and some mavic aksium or ksyrium wheels may have deprecated to that price point.
Be sure to measure yourself properly to determine the frame size you fit best on.
Similarly, if you want to build yourself, used equipment is the only way you can reasonably match the prices of the more aggressive vendors.