1/2 drive is the most useful. You can always use a 1/2 down to 3/8 adapter.
I would recommend one with no less than Sears Craftsman quality wise. One made in the US of by a good Japanese company. I have not seen an German or Euro made ones but if they are out there get a name brand one.
The more difficult choice is what torque range you need. I have a Husky 39104 click type that goes from 25-250 Foot-Pounds. So I cannot torque anything below 25 Foot-Pounds (I use my Beam type Torque Wrench for that or just tighten by hand).

My Torque wrench also has a Netwon Meter scale on it so I do not have to convert.

So you will have to decide how high you want your Torque to go.
You most often see Torque wrenches that go up to 125 or 150 Foot-Pounds but this may not be enough for some parts of Diesels.
I have been working on my suspension lately and the Eccentric Bolt of the Lower Control arm is torqued over 130 Foot-Pounds.
You might want to check some of the Torque values in the manual to help you dcide how high of a Torque you need.
Also there was a Thread about 3 weeks ago about Torque wrenches you might want to take a look there to.

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84 300D, 82 Volvo 244Gl Diesel