JoeCNC, from Ireland, cabinet maker.

[JAZZCNC]

Yes, a steel frame is a must if you want a good machine.

Don't start buying ballscrews or rails yet - it's far too early. Which ballscrews and rails do you currently have in mind?

100% agree here on both and just to add to it don't buy any electrics until ready to fit on machine and use.
I'd also recommend you buy has you go along and start with building frame, then order screws when you have accurate references to work from. Nothing worse than compromising the machine because you've been restrained or limited by components you bought wrong.

Going vertical will save loads of workshop space and it's easy to build cabinet around it to help contain mess, Think large wardrobe with clear folding doors and should give you an idea.?
Honestly no down sides to being vertical only upsides.!! Less space needed, longer cutter life due to gravity helping clear chips, better surface finish again due to gravity and not re-cutting chips, less mess(without vacuum) has high percentage of chips fall before getting slung around room and even then most hit sides of machine and fall to ground.

Has for design then don't copy mine has it wasn't intended for being vertical so can be made better if designing from scratch. It will soon be back on it's feet and up for sale after a re-fit with new screws, belts etc.

If your wanting purely wood machine then it's easy enough design to get right just needs correct selection of ballscrews and electronics to achieve correct cutting feeds.
Keep the base strong with low gantry sides or use the high side rail mounting you'll see on many designs on here along with shortest Z axis travel you can get away with and you won't have any troubles.

Look a around at other builds and come back or throw your own design together and ask advice. PM me directly if you don't want to do it publicly.

Good luck.