There are many self-repair books/guides available over at amazon.com along with some well-stocked appliance repair parts houses online. If it were me I would set both a dollar and a time limit for myself. Say, $200 in parts and four hours of my time. If it appears the repair will take more money and time than that, time to buy a new freezer! Also, better to purchase a manual defrost freezer for longevity. Only one issue, you HAVE to regularly defrost a manual defrost freezer. I use about eight insulated coolers to place the food items in while I defrost mine. If you don't regularly defrost your freezer, it will certainly wear out faster.

As far as brands to purchase, best to read freezer reviews on homedepot.com and lowes.com. From what my sources say, you should get at least ten years of life out of a refrigerator or freezer.