The buyer should present a irrevocable acceptance of loan financing, affirmed by a reputated bank before signing the notary contract. If you have a deposit, you should bring a validation of the funds.
If you finance with a local bank, this bank might accept a rough estimation by an reputated independent building expert.
If you ask for financing a bank, which is not regional, you would need a formal expertise by a "öffentlich bestellter und vereidigter Bausachverständiger" (You need a "Verkehrswertgutachten")

The deed of purchase and other accompanying documents (like the "Teilungserklärung" or "Baugenehmigung") should be checked with care.
If you are not german-speaking it is wise to have a professional translation done.
Of course the translator should be experienced with real estates and financing.

Real Estate financing can be done either with mortgage banks or with any usual bank.
Financing usually is done only for about 60-70% of the buying price by mortgage loan.
Financing for foreigners not resident in Germany and not earning their money in Germany is possible under certain circumstances. It will rarely exceed 60% of the buying price.

Financing should be secured before any contract of purchase is signed.
Sometimes one can sign a reservation or irrevocable offer for an agreed-upon period of time.
In this case you might look for your financing later.