Regardless of which broker you choose, you will have access to UCITS ETFs, many of which follow international indices. However, you cannot buy funds that are domiciled outside of the EU, at least not as a private investor. The website https://justetf.com has a database of ETFs and automatically applies availability restrictions based on country, and often has more up to date information than brokers. It also lists brokers that offer savings plans for each ETF.
All German brokers handle taxes automatically. You have a 800-something-€ tax-free allowance (Steuerfreibetrag) on capital gains per year, but you must expressly allocate all or part of this allowance to a bank (→ Freistellungsauftrag). If you make an error here, the proper tax can be calculated with your tax filings. Taxes are levied on gains like sales and dividends, but for accumulating funds also on part on unrealized gains. This means that the tax-optimal strategy is to prefer distributing ETFs until the dividend payments exhaust your Freibetrag. Beyond that point, accumulating ETFs defer more taxes until the sale, which is usually considered a good thing.
When comparing brokers you want to look at three kinds of fees:
- fees for maintaining a brokerage account
- fees for placing an order
- fees for using a savings plan (x € per month are automatically invested)
For all of these, Commerzbank is not very competitive with online banks, except at very large volumes. Often, banks like DKB or Ing are recommended in this context, but they can be choosy with their customers and I'm not sure if their websites support English very well. Flatex used to be very good until the recent introduction of a 0,1% p.a. account fee. Under no circumstances go to the local Sparkasse.
Rough comparison (as of Apr 2020, no guarantees):
- brokerage account fees:
- Commerzbank: free if at least one order or savings plan per quarter, otherwise 0,175% p.a. min 4,95€ ← very expensive for buy and hold
- Comdirect: free if at least two orders or one saving plan per quarter, otherwise 1.95€ per month
- Flatex: 0,1% p.a. ← expensive
- DKB, Ing: no additional fees, but associated checking account might not be free
- order fees (excluding exchange fees):
- Commerzbank: 4,90€ + 0,25%, min 9,90€
- Comdirect: 4,90€ + 0,25%, min 9,90€, better conditions during first 12 months
- DKB: 10€ if value below 10000€, else 25€
- Ing: 4,90€ + 0,25%, better conditions during first 6 months
- Flatex: 5,90€
- saving plan fees (per monthly occurrence):
- Commerzbank: 2,50€ + 0,25%
- Comdirect: 1,5%
- DKB, Flatex: 1,50€, some are free ← very cheap
- Ing: 1,75%, some are free
These fees are per order / per savings plan execution and usually depend on the volume/value of the order (except DKB). The number of shares you buy is not directly relevant.
What is more important than chasing the lowest-cost trades is finding a good, low-cost ETF that you are comfortable with, as mentioned use JustETF for that. Since not all brokers offer saving plans for that ETF, the choice of fund may affect your choice of broker.
A good resource for further questions is the German finance subreddit https://reddit.com/r/Finanzen