First, be prepared to pay extortionate prices for mobile services in Germany, compared to the UK.
Second, if you sign up for a contract, remember that you are locked in for 24 months, and that transferring to another operator requires you to pay a ransom to transfer your old number.
Third, and this happened to me on BASE so I don't know about other operators, for the first six months of contract you will not have any roaming service because you don't have a credit score in Germany so you can't be trusted with your phone.

That said, all the major operators (Telekom, Vodafone, O2) have an option where for 5€/month (again, locked in for 24 months and you pay also if you don't travel) you can use your data allowance in the EU and calls and texts to/from Germany are free.

Ask your employer if they have some deals with a specific provider (e.g. whoever provides their company phones) so that you can get better prices or better plans. The average plan with unlimited calls, SMS, and about 1GB of data (yup, 1GB) runs around 35€.

There are a number of other operators for PAYG, but AFAIK they do not offer any specific roaming package. Check our sites like this.