You haven't mentioned what you want to cut, but with a machine like that you will be limited. Wood and plastic would be reasonable if accuracy was not critical - e.g. engraving, or general shape cutting.

Beyond that there are many features which limit this machine, the high sides and single ballscrew being the immediate ones.

Looking at your lateral stiffness analysis this shows 1000N gives 0.39mm. This equates to about 2500N/mm. Although this would be a reasonable start point if you could achieve it, it's not clear if the force is being applied at the spindle centre line, or closer to the gantry on the Z plate. If it is on the Z plate then the real figure would be much less. This analysis is also idealised assuming ballscrew bearings etc. are all solid. Putting this analysis to one side I would say based on your comment about 'near perfect before buying . . . ' this needs more work on the general design before you worry about material spec (e.g. 5083 vs 6061).

Your other analysis showing maximum stress of 18MPa for 1000N is not that useful since all it shows is that the machine is massively strong enough and will not break during cutting. For example yield for 6061-T6 is about 240MPa and I think your cutting forces will be much less than 1000N. But don't confuse this with stiff enough because this will be a very basic machine as mentioned above.