Regarding the brakes, you should measure the reach needed from the brake mount hole to the center of the rim brake surface of a 700c rim, then make sure you get brakes that will adjust for that much reach.
The other issue might be rear dropout spacing. Since most older bikes with 27" wheels have freewheel hubs that require less space between the rear dropouts than newer cassette hubs, you likely have a frame that doesn't have the correct dropout spacing for a modern hub (assuming you're going to be using a modern cassette hub).
Bikes from the 1970's and very early '80's typically are spaced for 120mm hubs, and from the early '80's thru the late '80's or very early '90's, 126mm spacing was the common standard. Modern road cassette hubs are designed for rear dropouts with 130mm spacing.
The good news is that this is not by any means an insurmountable problem, especially since most older bikes have steel frames. If, for example, your bike currently has 120mm rear dropout spacing and you want to use a modern road hub, you will need to cold set the rear triangle (assuming your frame is steel), which means you will permanently bend the stays apart to fit the wider hubs. Instructions for this can be found on Sheldon Brown's website, under "frame spacing" I believe. If your frame has 126mm dropout spacing, you should be able to spread the frame by hand and squeeze a modern hub in since it's only a 4mm difference, but again if you really want to set it up right it can be "cold set" to 130mm-
edit: here's the link to Sheldon's site regarding cold setting a frame.
http://sheldonbrown.com/frame-spacing.html
And if you have to change the dropout spacing quite a lot (like 10mm), you should probably have your dropouts re-aligned after cold setting, the process does affect the dropout alignment. If you only change from 126mm to 130mm or something similar, it's not enough to be a problem from my experience.