Albert, does your CC currently have the stock 48/36 double crankset? If that's the crankset I believe it is, it should have studs on it to mount a third ring,...meaning you'll only need to spend a few dollars on the inner ring to convert it to a triple. You may not even require a longer BB. I couldn't determine from your post if you were shopping for a new crank. If you already knew this,
I apologize.
FWIW, I have a 48/36/26 crank paired to a 12-27 cassette on my steel road bike. When I throw on my saddle bag, pump and bottles, it's a full 30-lbs. I don't know what kind of hills you'll encounter, your present riding condition or the intensity/cadence you like to ride at. But I do know that I've not yet needed that inner ring. Even for my "lighter" riding, the 48/12 is a waste. If it was easy and cheap, I'd gladly change the 12t for an 18t cog. I wouldn't want an 11t myself.
If you do keep your 12-25 cassette (or go with the
12-27), and if you have no intention of going with a wider range cassette in the near future, you can use either a long cage MTB (SGS) dérailleur
OR a long cage road (GS) dérailleur, such as the Tiagra you mentioned. This assumes you're using a 48/xx/26 crank.
As for the brakes, get some quality pads if you're still using the stock pads. Also, take
a good loom at Sheldon's write-up on cantilevers. There's always the chance that your present brakes weren't optimally set up. If you still think brake replacement will improve performance significantly, consider just replacing the front brake and just fine tuning the stock rear canti. FYI, Kstop cartidge inserts can be purchased for $12/bike, so not big money to find out now. Also, Shimano has a nice canti for ~$35 (BR-R550).
Regarding wheels, Aksiums would be a fine choice for an affordable race or training wheelset, but they'd be about the last wheel I'd choose for commuting, LD rides or any type of touring. For about the same money, you could get a machine-built 32 or 36 spoke 105 wheelset and then add in a few dollars to have it tried & tensioned locally. Tiagra is also good for even less money.
If you still want a MTB derailleur,
here's a nice one for the
money.