Well garmin is the only way to go on GPS but not sure the price you are looking for I have found the 60c for around 370 new so look around or move up to the 60csx for a removable memory(don't need it unless you are going to yak around the world) they are water proof and tough as nails. There are others that are cheaper but 300 is the mark for a decent chart handheld.
The fish finder depends on what you want to use it for. (****warning my opinion only*****) fish finders are mainly used for find depth and structure so depending on the depth you are going to be at and or the type of areas, I would go for a garmin 90 to 240 and I think I am going to get a 250 color for my boat. Now on a yak you will have to worry about battery the 90 to 240 is a 100 to 800 (rms peek) now it isn't that high power but will last much longer on a 7a battery. The 250 up are 500 to 1000 and will eat one of those batteries up in just a little while. The 120 is a nice all around unit that I was going to go with in my aluminum boat and it is water proof (test is 3 feet at 30 minutes) so if ya dump your yak for some reason no harm.
bait tank there are several different ideas you can go with but for about 40 bucks you can make an awesome yak tank. I made this one for my aluminum boat but with a igloo cube instead of a 5 gallon bucket.
http://www.kayaksportfishing.com/equ...k/livewell.htm
the rest I will leave up to the yakers but let me know if you need a hand with that tank I have made four for other people I am getting to be a pro heheh.