I agree with Catherine on all points – getting a recording and knowing how the harp part fits in will help so much, especially if you have limited rehearsal time. Plus, it’s a wonderful work, and you will enjoy listening to it!
The Sanctus is definitely the trickiest out of all the movements, mainly because of the speed. Memorizing the pattern is the best way to learn this movement. The 6th Mvt, The Lord is My Sheperd, is a duet with the oboe. It is not hard, but the harp is very exposed the whole movement, particularly in the beginning. The 7th Mvt, Lux aeterna, is not especially difficult either, but takes a little more work than the other movements. You are playing triplets, but there is a lot of give and take in the vocal part, so you need to be able to watch the conductor for a little push and pull. The other movements are pretty straightfoward.
I have performed this piece many times, and I love it more every time I play it! You can really hear every note of the harp part, and it speaks extremely well. Have fun and good luck!