First, to anyone planning on an equity line for any reason: don't expect to necessarily walk into a refi later on, even subordinating the equity line is still treated as a cash out refi. It has, and will continue, to screw people up. There are limitations on cash out refi's, with the good companies offering the rates you want. This is fairly new. Don't buy a house with less than 20% down, get an equity line, draw money and expect to refi the 1st. It can, of course, be done, but there are LTV and CLTV requirements. CLTV is the total between the new 1st mtg and the CREDIT LIMIT (not balance) on the equity line.
House value: 100,000
1st: 80,000
2nd: bal= $3,000 but credit limit is $20,000
Means you are at 100% CLTV, which isn't the position you want to be in. Most won't even like that.
Anyway, houses that you buy that need NO work, will need work. A house that you know up front WILL need work, will need a LOT of work. Just like a car. Buying a house is a lot more expensive than down payment and closing costs.
Here's my advice: like you said, you can't even drive it until next Spring. Work you tail off, like if you had the car now. Take that $500/mo or whatever you would be spending (car note, extra gas, insurance, parts), and put it in a new money market account EVERY MONTH. Just like you had the car now. By the Spring, you'll need to borrow a lot less money. My thought is that this attempt will help you *realistically* decide if you can afford the car or not. Especially if you aren't sure yet if you'll also be taking on extra house and son expenses. Don't add a 3rd draw on your income until you've tested yourself with the 2 that are most important.
Equity lines are fine for whatever you want to spend the money on, but make sure it's worth the risk of being UPSIDE DOWN in your house. You can probably get a rate lower than on your 1st mtg, but most likely it will be more like a credit card than a loan. I.E. interest only payments. You may end up paying A LOT MORE for the car than if you'd have gotten an installment loan, since you could put off real payments on the car and spread it out over 10-15 years. To use an equity line properly, you have to have a lot of restraint and will power. If you had to sell your house, remember you'd oftentimes be paying 6% to realtors, a few thousand for house repairs, closing costs etc. Then pay off the 1st and 2nd. Then hope to make some money. Just be careful and think it out. Usually, patience will tell you what to do.