Was about to get a toy- then I did some research...

[JAZZCNC]

Going to show you some machines I've built that are all the same design but slightly different in size or Z-axis type/length etc dependant on usage but all share the same profile size. 45 x 90.
These machines were designed solely for woodworking and range in cutting area size from 600x400 to 1300 x 700 with a mixture of single or twin screws. However, all of them will happily cut aluminum as you'll see from a couple of pictures sent to me by customers.

Now I'm not showing you these and saying this is how you should build your machine but what I am showing is that this kind of profile size will easily do what you need so don't go stressing about if your 120 x 60 is strong enough or not. What's more important is how it's used ie: L shape gantry bolted together etc.
Some of these machines are used for business and running 12hr days 6 day weeks and have been doing so with minimal maintenance for several years.

Can I also strongly advise you to drop the round rail and go with profiled Linear rail. It's much much better and far less hassle. Don't spoil a potentially great machine for a few Euro more. You'll also get the extra back when you come to sell it as you'll be able to ask more for the machine and it will appeal more to those who know what to look for.

Also, move the ball-screw on the gantry to the top and motor below to keep the length of the drop plate to the shortest possible. Another thing you need to be aware of and one of the reasons why I use the Bosch Rexroth style profile with slot spacing at 45mm is that it suits BK/BF12 bearing bolt spacing. This means you won't have to make mounting plates or mess around drilling the profile.
It's these little details that can easily screw your ideas up or make things much harder than they need to be.!!