My own williams ownership has been pretty easy going, purchased in 2003 on 18.5k she was in near showroom condition, the fact it had spent most its life parked up has its own problems but a new set of rubber, and a wheel bearing and quick service soon got it right up to scratch! In total ive had no major problems and i find ownership cheap as chips compared to when i had a 172; Ive now covered almost 30k of williams ownership and apart from her age which sadly is something you cant avoid with a bit off looking after and a good valet it still looks as good as new!!
She's almost twelve years old now and total reciepts add up to just under £3800 on parts/service's, Think the only common wear/tear parts are wheel bearings, ball joints, disks/pads and steering column seems quite popular.
Beware rust although i would be very supprised to find one thats free from it thats actually driven on a daily basis, otherwise she should pull like a tractor and bounce off the limiter in 5th, handle like a dream and look generally slutish! He He.
I think i would only ever consider selling it for a immaculate ITR, S2000, EVO VII or (If i was to be all grown up) a VW TDI 150.
P.S my sunroof has never leaked