Had a nice answer to your post, this morning, Minx and then our local server went belly up before I could finish. Guess it's all part of the game with computers.

You are right to jump in, now. That gives you all summer to work up to that century in September. That's important with being new to road bikes. It takes time for your body to adapt to that road bike riding position, but it is well worth it. It's still the most efficient for putting in the miles. Just learning all the ways you cause drop bars is an education.

Is a WSD right for every gal? Hardly. Sometimes yes and sometimes no. It's been a blessing for me because of my proportions. I'm just a touch under 5'10", but it's mostly legs. I have a short torso average length arms. My perfect frame size is actually a 55cm, but those are few and far, between. That usually puts me in a 56cm to get the right seat tube height and crank arm length.

The problem for me with men's 56cm frames is the reach - distance from the seat to the handlebars - because a standard 56cm has too long a top tube and comes from the factory with too long a handlebar stem. Having too long a reach bows my back and that is a guarantee of back pain and discomfort on long rides. My usual fix is to go a shorter handlebar stem, but that can sometimes change the handling and not always for the good.

Along came my Trek WSD Domane 56cm and, for the first time in my life, a bike that fits with no modifications, other than adjusting seat height. can't say exactly why, but it fits like the bike was made just for me. The Domane is also what we call an endurance class road bike, with a long wheelbase and relaxed geometry. It really is a bike I can ride all day. The Domane will be out of your price range, new, but you might be able to find a used one.

But gals come in all sizes and proportions, so the right men's bike may actually be a better fit in many cases. How to know what's right for you? Spend some time with a pro that really knows about bike fit, specifically a pro that knows proper fit with road bikes. Find a pro shop and ask for the resident "roadie". Most bikes shops have pros that are either roadies or mountain bike gurus. Since fit on a road bike can be so subtle, taking to someone who lives and breathes road bikes is a big plus. They'll also have better advice on accessories and so on.

You touched on one of the biggies for bicycling health and efficiency, namely, proper seat height. Be religious about it. I often tweak the height on all day rides, as needed. Always carry a wrench and mark that seat post. Sometimes a half inch can make all the difference. Too low a height and you risk blowing out your knees and having back problems. Too high, and you lose power and put too much strain on your feet.

As for components, there was a time when I would settle for nothing under 105, but the new Tiagra is probably as good as the 105 group back in the old days. Should last you a long time. Anything less and you start to see issues with shifting and durability. The good news is there should be some Tiagra equipped bikes in your price range. Yes, you can switch out to a higher level group set, down the road, but that costs big bucks. You're dollars ahead to get something decent when you buy the bike.

In your price range, you can get any material you want in a bike frame as long as it is aluminum. That's not at all a bad thing. I did my share of centuries on aluminum bikes. Also agree that Giant gets you a lot of bike for the money.

Hope this helps.

And, welcome to the forum.