I avoid steel bikes like the plague. Certainly there is a cult of those that love steel, and they repeat the mantra 'steel is real'. I think if you say it long enough you start to believe it.
However, even the best steel bike leaves something to be desired. A classic Klein or Cannondale give such an incredible ride that's its impossible to compare to a vintage (or modern) steel bike. It would be like comparing your grandfathers truck to a Maserati.
Its not a popular perspective, but the reality is that aluminum, carbon, and titanium just build 'better' bikes than steel. The reason so many boutique builders work with steel is because its cheap. The skill level required to braze lugged steel bikes can not be compared to learning to TIG weld. Sure lugs are aesthetically pleasing, but they just don't build into epic bikes. A bike's soul is the frame and the wheelset. Starting with steel is starting with a compromise.
I don't avoid 27" wheels. These wheels have a 630mm BSD as opposed to the 622mm BSD on 700c wheels. It ain't much, but on frames over 66cm, of which plenty exist with 27" wheels, the difference in wheel size makes everything a little more proportional. After Continental stopped making the Top Touring 2000 tires in 27" my opinion is a little more qualified, but you can still get smart Schwalbe Marathons, Panaracer Paselas, and Michelin World Touring tires in 27". You can still get Continentals in 27" but they are more road race tires not the old touring tires I prefer for building up 'country' bikes.
Other bikes to avoid. At all costs avoid a bonded Vitus, whether it be an aluminum or carbon tubed bike. Every restoration and vintage expert I've ever seen comment on these has advised that the bikes not be ridden (at all) but only collected due to safety issues. Which is sad because they usually come with epic Mavic kit. These were great bikes, and very few are actually out on the road, and even fewer probably should be. Obviously the Viscount 'death' fork.
What else to avoid? Any vintage bike with an vintage alloy handlebar. Almost nothing can be as catastrophic as a complete loss of steerage on a bike at speed. Aluminum bars should be treated as 'schedule replacement' components on your bike just like your chain, rings, and cogs. The aluminum fatigues and after so many cycles they will fail. Its isn't a matter of if, but a matter of when.
Leonard Zinn says replace after four years of use, or one year of use if racing. Zinn has a background in physics, is the tech guy at Velonews, and builds his own components and bikes out of steel, aluminum, magnesium, and ti. Trust his opinion, he isn't just trying to get you to buy more bars.
Any bar that has hit the ground, either the bike falling over, or laying the bike down should be replaced in my book. The small diameter of vintage bars makes them especially suspect considering modern oversized stuff (less prone to failure).
I'd avoid anything that isn't equipped with a slant parallelogram derailleur. Until Suntour's pantent expired Campagnolo, Shimano, Huret, and Mavic were producing derailleurs that shifted much more poorly than the Suntour stuff. In looking to a vintage build know what it is, and why you want it. I don't have a problem with a vintage/classic bike being your only ride, but know what you have. With vintage stuff there are very real compromises...
I'm a huge Mavic buff, but I wouldn't recommend that anyone use the old Mavic brakesets. For my 'tout Mavic' build I'm using modern Mavic SSC brake calipers. I was pleasantly suprised to see that so many people preferred them to Dura-Ace and Campy Record calipers. Essentially a vintage bike should be avoided if it can't be ridden safely.
There is a boundary where keeping it period correct has to give way to safety. I think handlebars, brakesets, and tires are often overlooked on these bikes. I think that is a dangerous gamble. A blowout or a loss of steering on a technical descent or on a busy road could cost you your life.
To me that is the thing to avoid most of all...