Welcome to the forum.
Depending on the year you may be looking at a B6 or a B7. The B7 is clearly identified by the bottom of the headlights being curved (taller in the centre) whereas the B6 are reclangular and have a straight bottom edge. Other than that the cars are very similar (identical interior).
The 1.9tdi and 2.0tdi were never used in the B6 cab, therefore the 2.5tdi is the only B6 variant you will find unless you consider a petrol engine. I had a B6 2.5tdi (saloon) and it was a great car, but after having the HPFP fail on me I couldn't recommend it...
The B7 came with the 2.0tdi and the 3.0tdi, I have yet to see an A4 cab with the 2.7tdi although there are a few saloon/avants around, all be them quite rare. The 2.0tdi is a solid engine, however there are two variants, the 140 and the 170. The 170 (as you probably have guessed) has a higher power output, however this was subject to an injector recall which has been known to cause issues. So unless this has been rectified and there is a receipt from Audi, I'd steer clear of any 170's and try to find a 140. I have assumed both engines went into the cab, however I am not 100% on this.
Your other option if only looking at diesel engines is the 3.0tdi... I can wholeheartedly recommend this as an awesome engine and a brilliant combination with the auto gear box. More torque than an RS4 (when remapped), average 35mpg and they have very few problems. Only thing to look out for on a test drive is if there is any rattle (sounds like a clattering chain) when starting the engine. This should last for a couple of seconds while the oil pressure builds up, but if it remains after this time... walk away. If it's only there for a couple of seconds the cam-chain tensioner will need replacing at some point. I believe there are two and depending on which one has failed, it will either be a fairly simple job, or the entire engine will need removing to replace it, so be warned!
Having said all of the above, I've had a 3.0tdi cab from 108,000miles, now on 136,000miles and there is not rattle (even momentarily) and it's sailed through every MOT with no advisories other than wear and tear items. Always starts first time, brilliant car to drive.
Whichever engine you go for, if you're after low-end torque get a professional remap, don't bother with a chip. This will allow the turbo to spool up sooner in the rev range and give you the low end power you're looking for. It can also, if the power is used effectively through the gears, improve your MPG... however this is entirely dependent on your driving style!
Good luck in the search!
When reading these forums and looking about issues with different engines, it's always worth bearing in mind that most people who post on forums usually do so BECAUSE of problems, very few people pipe up to say that their car has been running fine for X years with no issues... so for every post about a problem, there are usually 10 happy owners with no issues!