Hello and welcome to the forum! I'm also new here!
The Rachmaninoff sonata no. 1 is a piece that is rarely played, and undeservedly so, I think. It's a great piece.
The third movement is quite hard, but if the Concerto no. 2 feels good under your fingers it might be fine. I would guess that the Sonata no. 1 is a bit harder, but it can be hard to rate these things. Why don't you give it a try, taking an xtra look at some of the places that look difficult in the score, and try them out a bit and see if it feels doable?