Road bike on a budget

Not sure how much you want to spend, but I would try to get a bike with at least 105 components.  Tiagra and Sora are really for recreational riders and the shifting is different than the higher group sets.

As far as the frame goes, I would suggest doing a lot of research before you purchase a "no-name" frame.  Anyone can build a frame, stick a slick paint job on it and sell it for less.  The frame should be your first priority.  It is the foundation of your bike and if it's bad, then nothing else will be good.  If you are going carbon, there are a myriad of carbon layups, construction processes, and types of carbon fiber.  Make sure the frame comes from a reputable maker.

There are a lot of bikes out there with Ultegra group sets, good frames, and decent wheels for realtively low prices that you could ride for years without needing to spend any additional money.  If you spend a little more now, you won't have any added costs to upgrade later.

No matter how much you spend on the bike/components, there is always a newer/sleeker/shinier/lighter/(better) option to upgrade.  People with Sora/Tiagra pine for 105's.  People with Ultegra dream about justifying the Dura-Ace components.  Even guys with D/A want the latest iteration. 

What is your temperment?  Are you inclined to "need" the latest-highest stuff?  If not, even Sora is serviceable and can give you years and years of service.  As for upgrading by parts, check Shimano's website as not everything is readily swapable.  Sora is often 8 speed (sometimes 9) and often has the option for a triple chainring (24 gears) which may give novice cyclists a little better range of gears for climbing hills.  As you move up to 105 and beyond, they are typically 20-speed (2x10).  Gearing on the floor bikes with 105/Ultegra/D-A caters to avid cyclists who ride enough to push a bigger gear up the hills. 

If you "need" the latest and greatest, just buy something and plan to replace it next year like your friends.  Unless you're competent to do everything yourself, you can't usually justify paying someone to swap components from one frame to another.  Yes, it gets expensive and you rarely get what you think the old bike is worth, but that's the cost of playing in this space.