I frequently have enquires from customers (and others) wondering whether to buy all new bike components (at bargain, online prices) or a new frame or a new bike.. These people are usually semi-serious cyclists with aspirations of moving up a category, going faster or looking for additional motivation to keep riding.
I recently helped a British man renovate his hand built English frame to All-new Shimano components, so he could keep it as his commuter/rain bike. His was an unusual case as he was keeping that bike-no matter what!
My normal response to someone that wants to go faster, or just change the bike -is - get a new bike!! Some cyclists spend hours riding with their compatriots, talking bikes and decide - usually after a few beers, that they are experts in selecting a bicycle component group. duh!! it just freaks me that they will make decisicons on the order of two thousand dollars, because "one of the guys he rides with likes his."
Bicycles are very complicated, non-linear systems that were designed by non-engineering types - to work!!
That's all -. no linear systems analysis; no acceptable design priciples; they built them, rode them, then cut away all the stuff that didn't work like it was supposed to. I've seen credentialed Scientists throw up there hands when I try to explain gear-inches or riding position or any part of what Wilbur and Orville Wright really did for a living. It's all derived from old stuff that worked -so now they make it better. lighter, therefore faster; but none of it works in computer modeled finite elements sense-it just works.
Product managers at all the brands spend many sleepless nights finding the right mix of components that will meet the performance spec's, but also the price point. How is joe rider- (who is an accountant,or realtor or whatever) going to be smarter about componets than these guys?
Not only are you at risk buying stuff online (id theft, bait & switch) but you have no recourse if the stuff isn't exactly right. Once you open the box - it's yours- and you get to make it work.  Further=any shade tree accoutant can tell you - you spend far more for the components in a bicycle than if you buy the package.--at least Sixty to 70% more...if not twice as much. Then you get to put all the labor into it- borrow/buy tools, etc. Pile that, on top of you really don't know what you purchased and/or how it works and it's a recipe for disaster that will leave a very bad taste in the mouth of some amatuer mechanic.
Get a new bike from a reputable dealer- no free lunch.