how much do you want to spend? you have the blichmann boilermakers in the 10 gallon size for a bit over $300 with a thermometer and sight glass. polarware sells 10 gallon kettles for less and there are many other options north and south of there depending on your budget. i started with a 7.5 gallon turkey fryer and made good beer.
how many brews under your belt? i ask because my suggestion would be to buy a cheap kettle at first to make sure it's going to be a regular hobby for you. then you can upgrade to a nicer kettle once you know it's a long term thing.
that said, you could get a very nice quality aluminum kettle from a restaurant supply store for a decent price... probably in the 50 dollar or less range.
Can I suggest getting a kettle no smaller than 10 gal? I use one kettle for strike/sparge/boil. I recently made a batch that required a 90 min boil, I had my 10 gallon pot full to the brim with water.
The tri-clad bottom is not necessary but certainly welcome. It takes quite a bit of heat to boil 7-8 gallons of wort. I suggest to get it if you can afford it but plenty of people have brewed awesome beer without it I'm sure.
I got my 10 gallon MegaPot from Austin Homebrew Supply - paid $119.99 about 1 1/2 or 2 years ago. That was with no valves, sight gauge, or thermo. - just the pot.
I've used Aluminum pots for about 40 brews now and they've been great. Used an 8 gallon one for a while, and recently upgraded to a 10 gallon, which I converted to electric. To my mind there's no need to go with anything snazzier - Aluminum pots work just as well as SS ones (search on the issue), and are easier to drill, modify, etc. Just my $0.02, though.
Something like THIS will do you great. Though that Adventures in Homebrewing one (here) is a good deal, too. Valves are always nice to have (I put them in my pots myself, but by the time you end up buying all the parts, it'll end up costing about the same).
Like one of the posts above me, Adventures In Homebrewing has a great pot for $79. Two welding ports for valve and thermometer. It has served me well. www.homebrewing.org they are a great group of people.
Save money and buy an inexpensive aluminum kettle. I got the Winco linked to above off of ebay for about $44 shipped. Then spend the rest on ingredients or other brewing gadgets. To me... more beer > nice kettle.
I have a 10 gallon aluminum kettle and love it for 5 gallon batches. Never worry about a boilover. But if it were only 7.5 or even 8 it would be close.