Yes well, a fool and his (her) money are easily parted are they not? Actually I hear what you are saying and may well start out that way.
I spoke to a professional photographer (no not the shop salesman!) and understand that the extenders can have a negative impact on the outcome of the shoot. Too many layers of glass introduce too many variables that are unable to be effectively managed.
I can pick up a relatively new 100mm lens here for about $NZ800.00, but I will focus on what I have for now as I have the EFS 17-85mm and the EF 70 - 20mm f 2.8L IS USM - so I have enough to learn on in the interim.
I was looking at the different tripods available and will probably get a fairly basic one to start with and it will need to be light so that I can carry it around at the racing track as I am climbing over pit walls etc. I had a look at the one referred to in this thread earlier and it looks keen. There are a huge number to choose from, I guess I just need to make sure that it has the right weight, height (both high and low), load ability (my camera and big lens are rather heavy), cost, and fits the right head (3 way so that I have more options for the motorsports.)