The TÜV is a must.
Mainly the TÜV checks the car for saftey.
Brakes, lights, ervry thing thight and no loose parts, no outstanding parts and so on.
Normally if one brings its car to the TÜV it costs about 90,-€ (varies locally), if the TÜV has no complains.
But as far as i know it exceeds this by far in most cases due to the TÜV has to register new parts in the papers.
Things get easier when you use parts that already have an approvment.
The electromagnetic tests are very expensive too.
In most cases you will be fine and done.
But you may run into some trouble:
In the cars licence papers it still will be approved as a car with a cumbustion engine.
Meaning you may get difficulties in case you want to have the tax exemption.
Tax and insurance fees will keep the same as approved in the papers.
In the case that the car is very old and never had a catalytic converter for exhaust cleaning, it may happen that you won't get the correct badge.
Thus you won't be allowed to drive in some environmental city zones.
No matter if the car now runs electric without any exhaust pipe, just due to the licence papers.
Relicensing the car and changing the papers may get very expensive.
But I am not sure about all this licensing process.
I highly recomend to ask someone with experience in converting cars.
Find some reports in this board here:
http://www.goingelectric.de/forum/umbau/