The 35mm is a brilliant lens. I've got one and it's stunningly sharp and I'd definitely recommend it. It's probably too new to get a second hand one at the moment.
You might find a second hand Sigma but they're pretty popular so they don't come up very often.
Oh, you won't have any compatibility problems with either of those lenses.
As for flashes, I've not yet got one so I don't have much experience but I would suggest bouncing the flash off walls or ceilings and turning the power down if you're suffering with the shots being too washed out.
There are loads of good places to get second hand equipment. I'm generally wary of e-Bay myself for some things but there are plenty of decent reputable second hand camera shops out there.
Try the following...
Mifsuds
Ffordes
London Camera Exchange
Harpers Photographic
Grays of Westminster
Ace Cameras