Ok, you want to upgrade your lenses but I get the feeling it's because you don't like the ones you've got rather than having a strategy of which lenses do you need. However, I used to own the 70-300 and rate it a cracking lens. Mine used to self extend until I contacted Canon and they replaced the whole optical unit, gratis. The only reason I sold that lens was because I needed an extra 100mm. If you don't want to spend too much I don't think you'll find a Sigma or Tamron that will beat the 70-300 for IQ in that range - and the IS is superb.
I can understand wanting to replace the 18-55, although I feel it is rather unfairly slated, and the 17-40 is an excellent lens. You mention a walkabout lens? I reckon Tamron and Sigma have it here and I use the Tamron 18-200 although their new 18-250 might be worth a look.
Whichever way, you are not going to get prime or L image quality withour paying those sort of prices.
But......it's all about what you need and want for the type of photography you do. My suggestion would be to sit down and think about the range that you need and then which lenses might meet your needs within that range at your price. If I might suggest - have a look at the range of lenses and reviews in our
Equipment Reviews section. Oh and....about filters, Lee and Cokin are both very good but which filters do you need?