If you actually looked at the hardware differences between a 136bhp engine and a 177bhp engine there is probably very little in it.
Usually the turbo but on a lot of modern cars there is nothing in it other than the map in the ECU. Take the 218bhp 3.0 V6 TDI it has exactly the same hardware as the 272 bhp car.
The towing capability of the car is not just the ability to pull the weight it's also got to stop it as well. I know the caravan has it's own brake system but the car brake system has to cope with increased loads when towing.
The standard brake system on a 136 bhp car will probably have smaller front discs than the 177bhp car.
If you are repairing peoples remaps every day then those people have had poor remaps.
Look at the parts listing for the engine.
http://audi.7zap.com/en/rdw/audi+a4+avant/a4/2013-736/1/
I have had a look and as I suspected the block, cylinder head, pistons, crank, con rods , water pump, oil pump, radiator and cooling system, EGR, intercooler , injectors , high pressure pump, etc etc are all the same on the 136bhp and 177bhp CJCB and CGLC.
The main performance differentiator is the turbo and the clutches are both 240mm but the 177bhp may be a higher spec to deal with the extra torque. However most diesel cars clutches cope quite happily with a stage 1 map.
The different turbos
http://audi.7zap.com/en/rdw/audi+a4+avant/a4/2013-736/1/145-145025/
You can pick up a second hand turbo off ebay for £250 plus
http://www.ebay.co.uk/itm/Audi-A4-B8-2014-2-0-TDI-Complete-Turbocharger-03L145721B/201734189608?
. or a brand new one for £600
http://www.ebay.co.uk/itm/NEW-GENUI...URBO-TURBOCHARGER-03L-145-721-B/400904338320?
With the turbo upgrade and having a remap I would probably go for somewhere around 190bhp, why stick at 177bhp when you can have a bit more easily.
So you don't need a new complete engine, change the turbo and if you want change the clutch and of course get a remap.
You will probably have 314mm brakes , I would be tempted to upgrade to 330mm if doing this.
If you are anywhere near Stockport Manchester I can strongly recommend Rick at Unicorn Motor developments he will sort the car out for whatever you want remap wise.