New, the Clarke CL430 has 25mm through hole.
I've had a CL500 for nearly ten years now, which is the 430 with a milling head, but the milling head was only ever on it for a few weeks, and it's done well in the time I've had it.
I spent some time comparing various new lathes earlier this year, and the Clarke was the only one in it's size range that came with a 25mm through hole, everything else was only 20mm.

Second hand, you will pay a premium for smaller lathes, but you've got to be wary of what you're buying.
Colchester Bantam may be more suited to your space requirements than a Student.
Boxford xUDs (I can never remember what the first letters mean!), Harrisons are good makes.
Other option is check out Warco/Chester to see what sits in yoru size range and see if any are available secondhand on the usual places.