For changing the oil:
http://www.986faq.com/7-0/oil.asp
There is a lot of good information on that website. You can get to the engine from the top too, there are special instructions on how to do that.
They always say winter months are better to buy convertibles, but I don't agree with that statement. Then again, I live in San Diego and the weather is pretty nice year round. If you are really looking to get the best possible deal, it's more luck than anything else, and knowing what you want. I got what I consider an extremely good deal on mine, and I bought it in the month of August. I really don't think that Ebay has the best deals, it is just enticing to see a low starting price, and it causes people to bid up things the last few minutes/seconds before an auction ends. In 99 they made the gas tank bigger, in 2000 they made the base engine more powerful and introduced the S.
Welcome to the site, and good luck finding your Boxster!
Larez2