I would disagree with the "made from castings" commenet – apart from the table & maybe the underslide this appears to be largely fabricated to me.
On the view of the RHS of the machine (if you were stood in front of it) the welds can clearly be seen on the column, base & over arm. The overarm just appears to have a flat base sat on the flat column top, secured with a large bolt through a slot in the arm.
The Clarkson collet chuck looks to be of the 2MT type, with the threaded support collar & a small gap can be seen betweeen it and the spindle nose. I would think that there is a very good chance that the spindle is 2MT. Even if the collet chcuck is "built in" to the spindle, fitting other tooling would be possible using parallel shank tools in the collet chuck.
There does appear to be some movement capability to the spindle, as gib strip screws can be seen on the LHS of the front swivel slide behind the spindle "casting" & on the side view from the LHS of the machine there is what appears to be a leadscrew behind the spindle "casting". Can't see from the pictures posted how this screw would be turned, though, and it may be more of a fine spindle downfeed than useable for drilling.
The table seems rather narrow compared to the knee & I wonder if the builder used the table & underslide from a small horizontal milling machine & fabricated the rest to give a larger machining envelope than just putting a vertical head on a horizontal mill ?
Not sure why the seller calls it a horiziontal mill, though, as I can see no evidence of a horizontal spindle.
Nigel B.