But i've now started thinking about the possibility of using aluminium profile instead.
living in the damp old uk it makes a lot of sense to me to upgrade from mdf

i cant see any reason to use 80/20 if you cant find any... there are lots of other t-slot profiles and suppliers, i bought from valuframe (kind of pricy but they had the sizes i was after)

i'm wondering about the suitability of these for the job in hand & whether there are any plans or guides out there using these profiles
it might be worth finding a build on this forum that you like and twist the builders arm a bit to get him to give you his drawings and guide you through any of the pitfalls he experienced (what kind of cash are you willing to splash on rails and screws?)

also would the resulting machine perform better than an mdf one?
on a like for like build id buy myself a hat and eat it if it didnt

if you copy a design from this forum you will be able to see the kind of work thats being done on that machine