I am a citizen of a non EU country with an offer for a PhD position in a research training group with a TV-L E13 (65%) salary. I was told getting a student visa would be problematic. Which visa would I require to work under TV-L salary.
A student visa is definitely wrong (and something you can't fix afterwards!) You need a visa for a long-term stay that allows you to earn money; this is usually called a "national visa type D". This will allow multiple entries. Be aware that it will usually only be issued for three or six months (I've seen both), and you need to get the "real" visa for the full duration of the contract ("Aufenthaltserlaubnis") at the foreign office ("Ausländerbehörde") once you have arrived in Germany.
To get the visa, you need to provide some documentation about your employment in Germany; specifically,
- date and place where the contract will be signed,
- duration of the contract,
- exact description of your position,
The university should know about this and provide you a letter of confirmation In your case, something like
$NAME, born $BIRTHDAY, is supposed to be employed from $STARTING_DATE to $ENDING_DATE as a wissenschaftlicher Mitarbeiter (TV-L E13) with 65% of full working time at the $DEPARTMENT of $UNIVERSITY.
would suffice, since it's a standard position the embassy will be familiar with. You also need to present your Masters diploma to verify that you are qualified for the position.