I would look for something with pretty close tire clearance for 700c tires (radially), and avoid anything that was built for 27" since they would need brake reaches longer than are readily available to reach the rims in most cases.

If you get something built for 27" wheels, you are probably already getting a bike that by todays standards needs long reach brakes, and then you'll need an additional 23mm or so of reach to get to the braking surface. For 700c, you are looking at 19mm, so even there, you should look for a bike that has minimal brake reach, so you are probably limited to bikes with pretty tight geometry to get the brake reach you need.

If the bike you are considering can take fenders with 700c wheels, you probably won't get the brake reach to convert to 650b without using drop bolts or some other "trick".