Tele converters are not great and cheap ones are a complete waste of money.
I have used (borrowed)the Canon 100-400 lens and it is excellent but very expensive.
I have a Sigma 170-500mm and it produces superb results-only slight downside is it is a bit heavy.It doesn`t bother me because I find its weight is helpful to reduce camera shake in windy conditions.
My suggestion is to pay as much as you can afford too-with DSLR`s you really do get what you pay for.
If you just want to take snaps a P.A.S is fine but too get really good shots you really need to go for a DSLR
A good site I have used in the past is
www.warehouseexpress.co.ukJessops and Camulet tend to be pricey and try to flog you extended warrenties which are very expensive and almost always useless.
Ebay is s good source for camera gear but beware of the Hong Kong sellers-you WILL get hit for IMPORT TAX no matter what they tell you.
Look for UK sellers with good feedback-golden rule of buying anything ........
IF IT SOUNDS TOO GOOD TO BE TRUE,THEN IT PROBABLY IS