Depends on how much you want to invest and what features you would like.
Since the average vehicle owner only needs one for infrequent use, I would suggest a battery powered hand held one similar to what beww shared in his link. You really do not need one of the caliber used by motor vehicle service shops.
The important point though is when buying, try to purchase based on solid customer reviews, and pick known reputable name brands if you can. Example: if using Amazon as the merchant sourcing, be sure that the composite of 3, 4, 5 star reviews from customers averages 85% (ie: very few 1 and 2 star reviews), that there are plenty of reviews (ie: not a low volume no name brand or a brand new product listing), and the shipment is either sold by Amazon the company, or fulfilled by Amazon shipping services (as this insures fast prompt service for any issues with a new product). It is also prudent to read through the 3 star reviews as they most often give you the most objective critical review comments. The 1 and 2 star reviews are most often buyers who simply purchased the wrong thing for their needs, or did not understand how to operate the product, or the product received was simply defective (Amazon will exchange or refund for free).