You need to ask yourselves if you're actually ready for parenthood. Read parenting books, go to parenting classes, and both of you see if you can baby-sit some kids of varying ages. In keeping this child, your lives will change forever - this job you are planning on taking on requires immense patience and a lot of money, among many other things. I'm not trying to talk you out of your decision, but please keep in mind that love and a minimum-wage job at mcdonald's alone will not provide an ideal life for a child.
As far as telling the 'rents, prepare for it, because it won't be pretty. Your parents will be disappointed, frustrated, angry, and probably tell you to move out. They may or may not follow through with their threat, and I advise you to brace for a harder impact if your parents are religious and/or believe in abstinence until marriage. Also, do not go making promises to them you can't keep...For example, don't promise you'll take care of the child all on your own if you have even a tiny inkling in your mind that you'll need to borrow/take money from them at some point post-birth.
The basement room actually sounds rather nice, and believe me, you don't need to have a tank for a vehicle just because you have a child. Something that runs, preferably on little fuel, would be just fine. I advise against a gas guzzler because you'll need all the money you can get for your child, so there would be no point feeding tons of cash to a huge car when you could get one that's smaller and uses less gas.
All I can say is you've got a few months to decide on your future, and I would advise doing more than reaching for straws - in a few months, you'll have another life depending entirely upon you and your girlfriend, and this should not be taken lightly.