2m+ travel lengths with a ballscrew, using linear rail to support the ballnut

[Desertboy]

I was thinking these
https://www.ebay.co.uk/itm/15mm-line...cAAOSwNWxaJP-~

I could butt 2 together to get lengths I desire so £240 would support 2 3m lengths.

The choices so far
1. 2* 2525 ballscrew 2.7m with decent quality BK/BF fittings ~£1000
2. 2* 3232 2.5m with fittings ~£1500
3. 2* 2010 with fittings 2.7m length fully supported with the clone rail and 2 carriages per ballnut leaving me 4 spare carriages in total, I could then use 6 carriages to support the Z axis which has the same rail on it given me 50% more strength without affecting travel. ~£600

I would think the 2 biggest factors in critical speed are how straight is the ballscrew and how good are the bearings in the BK/BF fittings, I might even play with type C mounting method but I believe this needs super precision although doubling your criticial speeds.

Resolution is not so relevant as I'll use servos, not sure how powerful but was told 400w should be ok I think £2k will buy the servos and the mesa controller so I can use linuxcnc.

I really want to direct drive with oldham coupling but it can be belt driven I suppose, the most important things are large travel, good accuracy and good speed.