Ed,
I took some time off from the piano for about 3 or 4 years (This was maybe 25 years ago now). It was EASIER to get back into it than I thought, so by all means, go for it!
I don't know grade levels, but you should probably start a little below that difficulty level, or you may get frustrated. You can work your way up pretty quickly.
Maybe try the Bach Little Preludes and Fugues before going back to the Inventions. What about Debussy's Arabesques, or perhaps some of Chopin's Preludes or Nocturnes? (Some are treacherous, but others are not so difficult.) Mozart's little Sonata in G (K283) is lovely, and not too difficult. Or K330 in C Major.
I am not familiar with an easy Mozart Piano Concerto! You might look at No. 12 (K414) or 13 (K415). For now, I would skip Nos. 14 and up.
I played Beethoven No 1 back in college--Love that piece! It is pretty challenging, though quite straightforward interpretively. Technically, the rondo is the hardest to master, as it has tricky phrasing in the main theme, crossed-hands leaps at rapid pace, and the coda has that rather exhausting series of parallel scales!
Have you played Beethoven's Op 79 Sonatina? Fun, and a nice preparation for his more difficult stuff.
Hope this helps.
Teresa