@ Frank braised is usually better as if you need a clamp the right size clamp is in my shed somewhere...if not ebay is a cheap place to get one
Peter how about asking Noel at bike station as everything he has is for sale :) Whatever you end up with make sure you have the bottom bracket to suit.....worthwhie to spend the extra bucks on that component....but I do have a spare FSA and DuraAce lying around here somewhere you can have for nix if that helps:)
I found that the 34/25 felt about the same as a 39/27 so if you are looking for a cheaper option you could try a 12/27 or 12/28 cluster first. Assuming you haven't already.
You should be able to find a gear calculator on the web somewhere to check. Pretty sure there is one on Sheldon Brown.
I run a 12/27 with my std crank all the time now (live in the hills).
I have an Ultegra compact crank that I bought for the mountains in Europe but I don't use it at home as I found I had to change the front ring too much in normal flatter riding here. Not for sale though as I have some more mountains that I want to climb.
I've got a 105 compact crank in the shed, I think, came off the last bike.
They are all pretty much the same. Ive riden them all from tiagra to DA/Sram Red and cant tell any difference if I close my eyes lol!
These days the crank technology is so similar its no real difference.
Get your bike fitted by an experienced fitter as well if you knees are a bit knocky. Mark all your changes with a liquid paper pen so if anything slips, you got a reference point to go back to.
and the rollout of a 50 x 11 gear is actually bigger than a 53 x12
will your bike handle three chain rings at the front?