Years ago I ordered some BK bearing blocks and they did not come with the spacer tubes (2off). What happens is the ballscrew gets clamped up hard against the seals (the parts left inside your blocks with the bearings removed) and they are then very difficult to rotate. I don’t know why they sometimes send them out like this.

I had a friend turn up 2 spacers on his lathe and all was well.

Either ask the supplier where the spacers are
Or get someone to make up some spacer tubes

If you check my YouTube channel (routercnc) episode 5 shows me building up a ballscrew and mount. Note the brass buffers are nice to have but could still work without. Here you will see the spacers you need.

In terms of bearings then you have the budget ones. As Clive says you can optionally upgrade to AC bearings but either way you need the spacers.