I'll try my best to help you out Martin. I am answering on my
experiences and what I have learned along the way. I, too, have an
Einzelfirma and have looked into GmbH a while back.
1. That is the Gmbh Light... 20k capital. Most start that way these days.
Earlier 100k was necessary. Yes, that stays in the company.
2. Probably...
3. Not that much. I pay out 2500 a year plus MwSt for me, myself and I
with less than one binder of in and out invoices per quarter.
4. As GmbH you are an employee. Not the same with Einzelfirma. You
still pay 100% either way

5. Check with the branch you work in. I found a good one that runs me
over 700 per month, again 100%. Insurances are a must, my suggestion
is to find an independent insurance handler, one that offers
bookkeeping and accounting in their list of services. Get one company to
handle the finances and insurances. Reason is, they have to talk to each
other anyway with regards to your income and insured value, etc.
6. That can wait. There are advantages to being self employed, you can
put more into this fund per year, well over the 6k+ maximum that is
allowed for employed persons.
So, if I am wrong, sorry. Someone will surely correct me if so

Good
luck, keep us informed and share your experiences with the rest of us.