Help with Unit factories

I am trying to make a Factory building where you can pick a unit to begin building and queue up, 1 unit type at a time, no 2 different units in 1 queue.

However i dont know the best way to actually make the “pick unit” part, i’ve decided on it only working with no units queued but im not sure how to actually make a gui which isnt cluttered and can scale indefinetely with new units being added

Ive tried Surface Gui scrolling frames, but if i run out of space on them i have to resize everything to fit more so i decided against it, What is the best way to doing this which can easily be scaled up?