Can I beg to differ on the idea of necessarily needing a shorter first/middle name to go with a longer surname. I gave this a lot of thought when I was pregnant, and I believe that it's how the names 'scan' that matters. My dds and dh have a long (3 syllable) surname stressed on the first syllable.
My dd1 has a three-syllable first name also stressed on the first syllable. So if she says her first name and surnames it 'scans' (like the 'feet' in a line of poetry): DUM-di-dee DUM-di-dee. Her middle name has two syllables and she also has my surname (2 syllables) as an additional middle name, so altogether it would be DUM-di-dee, DUM-dum, DUM-dum, DUM-di-dee.
I did want to keep the same rhythm for dd2, but we fell in love with a 2-syllable name for her, so we gave her the three-syllable version as a middle name instead (still with my surname as an additional middle name). Hence she is DUM-dum, DUM-di-dee, DUM-dum, DUM-di-dee.
I've just read all that back, and it does sound a bit mad, but I really do think that it works if you think about the rhythm as much as the sound.
Anyway, FWIW, I think that Sarah is a lovely name and (unlike when I was little) quite un-common these days. Sarah Lucy Dum-di-dee works quite well. Or (going with my theory... this is assuming you don't think I'm a complete fruit-cake ) you could look for a three-syllable middle name to echo your surname... e.g.
Sarah Bethany
Sarah Briony
Sarah Lilian
Sarah Juliette
Sarah Isobel
Sarah Annabel
Sarah Abigail
OK, I've wittered on enough, and am starting to wonder whether my baby-naming technique might be just a teeny bit strange! Good luck, anyway.