Your best bet is a 3/8 (you will rareley need a 1/2 for normal maintenence). Go with a "CLICK" type Tourqe wrench in Ftlbs. They are the most durable, most accurate and easiest to use, set it, when it clicks, stop. I personally dont like the "Beam" style wrenches. They are not as accurate and too large for all applications. , they are also more easily knocked out of spec. "Dial" style wrenches also are sensitive and you have to look at the dial to know when to stop. As far as what brand, It really depends on how often you will use it and what you are using it for. If your only use will be torqueing your drain plug and maybe another bolt or 2 once a year, Craftsman will do the job. Its crazy to spend $400. 00 for a Snap on you will rarely use. If you have the money, and will be using it often, go for Snap-On, Proto, or Mac. Very pricey, but very accurate. I do alot of mechanical work on an almost daily basis, so i have use for my 1/4, 3/8, 1/2 and my monster 3/4 Torque wrenches. I also do alot of assembly work where exact torque specs are critical. Oil drain plugs, Alternator brackets etc, are not going to fail if your wrench is not Super Accurate. Just use your common sense on how much to spend and how much it will see use.
Whatever brand you buy, make sure you get a hard case for it and treat it with great respect. Always set the Torque setting to zero when finished with it (click type). Aother way to preserve thier calibration is dont use them to start long bolts, use a regular ratchet to start tightening nuts and bolts, then once snug, use the Torque wrench to tighten it. Also, using extensions decrease the accuracy, avoid using them if you can. The longer the extension and the tighter the torque setting, the more off the reading will be. Using adaptors that go to 1/4' or to 1/2" also throw the accuracy off. Also, forget using a universal joint adapter, they will totally throw off the setting. And lastly, DO NOT break nuts and bolts free with your Torque wrench, very bad for them. Good luck in what ever you choose.