Originally Posted by MichaelW
Go for current wheel sizes, 26" MTB or 700c, not 27".
Unless you need to go for Kevlar tires etc like Michael W mentioned later in his message, I disagree with this statement.
27" is still widely available as long as you don't need the latest high tech tires... and having them reduces the theft factor because some people see them as unavailable. The fact is, I can find 27" tires more places than I can find 700c tires where I live. All of the *Mart stores carry 27", and none carry 700c. Bike shops tend to carry both. You won't find many of the latest $80 tires in 27", but you will find plenty of sub-$20 tires for them.
I would not suggest specifically going for 27", but don't bypass a bike that otherwise meets your needs if it has 27" tires. I just put some nice Continental tires on my 27" wheeled touring bike... and a set of 27" tires on a beater that cost $5 each.
Oh, and if you do buy an old steel rimmed bike, I would suggest swapping the wheels for alloy rims for wet braking... If you get to that point and need a set, look at the thrift stores and buy a bike with the right wheels for $20 and swap them. Or ask in the Classic and Vintage forum here, someone may have a set they are willing to let go of in trade, or cheap... I bought a nice used set for a total of $40 (half of that was the shipping).