If you are only using NSX-T DFW. you can prepare ESX hosts with security only. where using NSX-T you can do DFW for VDS port groups. But keep in mind security only will not provide networking and overlay functions. only DFW will work.
If you would like to use NSX-T for more than DFW, please prepare ESX hosts with Networking and security. where you can use DFW and Networking, Overlay, basically all NSX feature set. But DFW will not work for VDS port groups. and you need to move your VM's to NSX-T Overlay or VLAN backed segments. keep in mind you don't need to have N-VDS. you can prepare ESX hosts with existing VDS switches as well, which is called as Converged VDS(C-VDS) (Yes, in this process you need to configure uplink profiles and transport node profile). and hence don't need to bother about limited uplinks and downtime. once you prepare ESX with existing VDS. NSX-T VLAN Segments or Overlay segments will appear under that C-VDS and you can move VM's from VDS port group to NSX-T segments under same VDS. this VM's movement shouldn't cause any big downtime rather than a ping loss. and host preparation will not cause any downtime.
Hope this helps!