I use mix and match
from 9 speed 105/Ultegra/Dura-Ace. So no clue about 10 speeder...
I simply follow the rule of replacing parts when needed with the available deal according to my degree of weight weenieness... (!!!) Do you know the difference between an Ultegra and 105 crankset? (please don't tell me, dear reader, that you "flex" my cheap 2001 Octalink 105 cranks!)
For example: I don't think my front derailleur is really wearing out. And 105 is definitely plenty good for this purpose. But I can find great deal on D-A cassettes that will save me lots of weight for the money.
This way I ended up with 105 cranks, brakes, shifters and levers. Ultegra bottom bracket, training cassette and hubs. Dura-Ace racing cassette and hubs. SRAM PC99 hollow pin chain.
How does this relate to your question? Well: 1) Shimano 105 is plenty enough for racing. No complains there. 2) you could start with this gruppo and eventually upgrade the parts that count. 3) don't feel insecure because of the material you use.
When you are limited on the funds available to ride: better ride lots and find money to get to the races than to spent too much for a gram here and there and a crisp shifter, only to ride around the block on top gear! (not to say people who swear by top gear don't ride... you get my point)
I am far from rich, but I can afford dura-ace all the way. Yet, I find it ridiculous for the price difference compared to the excellent performance of "lesser" gruppos. I prefer to change my cables and chain often (2500-3000 kilometers for a chain, 12 months for cable set).
Plus: you can always blame your cheap gruppo when you climb into the broom wagon. Can't do that on D-A can you?