First of all you should match specifications of both, one the machine which you are going to purchase and second of your job, you want to do on the machine.

Centre Height :
Centre height of your machine must be match with O.D. of your job.

Spindle bore :
Spindle bore should be with such dimensions that your job O.D. may be pass through it easily if it is necessary.

Width of bed :
More Width of bed gives more rigidity and sturdiness to the machine. In heavy work it is necessary.