It is exactly related with the requirements.
1-Table size: Should adaquete to fixture dimensions that you want assamble.
2-Spindle taper. Hsk type spindles will give you high presicion on run out and high turning speed but they are not strong enough against a collusion as SK types. It is a quest?on of is your process requires high speed or high power?
3-Coolant system. What is the minimum size of the internal coolant hole that you have in the middle of your tools. What is minimum pressure for that tool? Usually it should be at least 70 bars.
4-Vertical or Horizontal? big question Small parts, mqss production and 4th axis requirement signs horizontal. Big parts, big table requirement means vertical
5- machine accuracy? what is your part requirement as a tolerance? Or Consider the capability study.
6-Spare parts. What is the max time during a break down? Is it easy to find spare parts and service?