I would agree that for less than £200 the Sigma from Onestop for £116 takes some beating. I've been using it for 2 weeks now (about 800 photos) and while I'm not completely blown away (like I was with the 50mm f/1.8) it is obviously a solid lens and amazing value.
Most of the others that you list come off much worse in tests and some feel much worse when I had a quick play in the shop.
If you want to budget more, then IS would definitely be only my list of priorities as about half of those 800 photos have been binned because of camera shake
As for filters - you could order one of these at the same time:
http://www.onestop-digital.com/catal...roducts_id=294
It's the best out there.
Or for a bit less there's this
http://www.onestop-digital.com/catal...roducts_id=139
You can just leave these on all the time.
I wouldn't get a polarizer to leave on all the time for this lens, because the front section rotates during auto-focus (most annoying).
Re: other lenses to partner it. I'm just going to keep my kit lens for a while and get used to it. The other two I'm thinking of is the Tamron 17-50 f/2.8
http://www.onestop-digital.com/catal...hp?cPath=24_29
Or if I suddenly come into money - the Canon 17-55 IS f/2.8
http://www.onestop-digital.com/catal...roducts_id=341
I don't mean to come off sounding like an advert for Onestop, so you should way up warranty, proximity (they're in Hong Kong), price etc before jumping in.