At 41 I have done my time wearing stuff that *really* doesn't fit. Never again will I be in high-waisted pants that give my long-torso'd self a wedgie or tops that (accidentally!) show too much.
I use a tailor -- and lately have moved into heavily customized or made-to-measure clothes -- for all of the reasons you listed. While I don't sew, I actually started to read a lot of sewing blogs to help me better understand clothing construction; I find this helps me be realistic about alteration/construction cost. Also it's great to see the creativity out there!
[Would that I could get everything that was a little off -- even the inexpensive things like T shirts -- to the tailor, but budget realities (aka my greed for new stuff) intrude.]
Anyway: Because so much of my clothing budget was spent at the tailor, I actually made an effort to find local shops that produce their own clothes. This way I can start from scratch or get them to make changes to the pattern for things they can't alter after the fact...and the price point is in line with mainstream shops like Ann Taylor/ATL/Banana Republic/J Crew.
While this means my wardrobe is pretty small (and babied so that things last!) I am so much happier this way. It helps that I am much more of a classic/vintage-esque dresser than a trend-based one.
ps I'm pretty good about taking new stuff (or stuff I definitely want to change up) to be altered, but drag my feet over repairs!