question 1 what brand of radio will last the longest
Kind of like asking what will last longer a Ford or Chevy or Mopar? It's very hard to determine. I'd suggest looking where you want to talk and read up on that band, and see if you can find brand and model numbers that repeatedly get good reviews. Speaking of reviews, search a model # and eHam usually has doeens for you to read through.
question 2 which bands are busy
All of them, really depends where you are though too. I know around me 2m 6m and 10m almost always have conversations going. Search google for HAM repeaters in your area and you should find several 2m/220/70cm repeaters, but check as this often varies by location.
question 3 what kind of antenna should i install
One that is the proper size for your wavelength, as far as yagi, diploe, vertilce, loop, etc. that all depends on what you want to do with your radio and where you'll be putting your antenna.
question 4 where do you buy antennas
Most vendors selling HAM radios will have antennas you can buy, many antennas have designs widely availible on the internet and there's books (often in stock at the local library too) that can provide instruction for building your own antenna.
Generally with the questions you've got I'd reccomend a mix of referring back to the technician handbook and netowrking wiht members of your local club, this will also let you see HAM radio in practice, which I understand as a hard thing to make sense of until you see and hear how it all comes together.
