New tyres - GroovyMucker
Having managed to blister another tyre, I ended up going and buying two 215 50 R17 V Goodyear NCT5s. £102 each all in, which seemed to be reasonable from what I could find, and a helpful fitter who pretty much insisted he was going to swap things round so I had the two new ones on the back (which I wanted anyway).

After my cynicism about soul08, I suppose I'd better not say which Durham garage did it! Oh go on then, Tyre Serve on Renny's Lane.

New tyres - Dynamic Dave
Same sized tyres as fitted to my Vectra-C. I saved a packet by fitting an alternate size that is both suggested in the handbook and recommended over on Vectra-c.com - 225/45/17, as the size is more common than the 215/50/17. Might be worth checking your handbook to see if the same applies. Personally though I was glad to get shot of the NCT5's; they may well suit your Avensis but the handling was appalling when they were on my Vectra.
New tyres - GroovyMucker
Pardon my ignorance, but would I then need new wheels?

New tyres - Dynamic Dave
Nope, as the tyre radius remains 17". The only things that will alter are side profile (marginally smaller), and the width (marginally wider).