Estimating the renovation of existing properties including funding

Am I correct in understanding that a brick-faced solid construction might possibly have the advantage of inexpensive "injection insulation"?

Very unlikely, but possible; in your case, I would not plan for it. Cavity walls with an air gap were built until the end of the 1950s, after which, in brickwork, only a mortar joint was used as a separating layer until the late 1970s, and since then always with insulation in the cavity. In northern Germany near the coast, air gaps were still sometimes used into the 1970s, but rather rarely. In individual cases, this can be recognized on site with a trained eye, sometimes also by the bearing point of the wall plate, if the air gap is open upwards to the cold roof space or to the side space, which was sometimes done in the 1950s.
This is called "blown-in insulation," by the way.

Is it realistically possible here to achieve insulation of the facade, roof, basement, and replacement of windows to EH85 standard?

With respect to the requirements for the building envelope, very likely yes. You probably will not achieve the primary energy demand for EH85 without the use of renewable energies (heat pump, solar, water-heated stove) – this needs to be calculated.

- Would this realistically be possible for <100k€?

If contracted to companies: No.
If 100% self-performed: Yes.
But since you probably will not replace the heating system yourself nor redo the roof covering yourself, then likely not.
If it suffices to insulate the roof from the inside and the covering does not have to be renewed—which is unlikely for a building year 1970—maybe. But even then, only with great difficulty. A new roof with associated work costs €200/m² and up, the facade €120-160/m², windows €600-900 each + front door + sliding elements on the terrace if desired. But electrical work is not yet done, interior plaster/paint is not renewed, floors not renewed, the kitchen is still old, and the bathroom with old pipes is still there.

There are cheaper alternatives for many tasks, sometimes with drawbacks (internal wall insulation, internal roof insulation), and some can be done as self-work. However, your questions indicate a lack of experience in this area, so I would not recommend it.

If I now find a house that can be renovated with reasonable effort and that has an old heating system, could I then plan a budget of €150,000 for renovation + installation of a heat pump via a low-interest KFW loan, right? (assuming Level 85 EE is achievable)

has already written a lot.
I am currently finishing the renovation of a 1960s property into an efficiency house 55 (calculated between 40 and 55) and can say: Yes, it is relatively easy.
Reasonable effort? You have to decide that for yourself. I started just over two years ago and am currently well below €100,000 (for 200 sqm), doing almost everything myself. Today it would be significantly more expensive, the heat pumps alone (I installed two, as there are two apartments in the house) have risen about 67% in price online. The Resol insulation (ETICS) currently exceeds all price expectations – for 120mm (needed for 55 – I installed 160mm) it currently costs €75 per sqm. You can easily calculate the costs for the facade using just the insulation boards without additional materials and installation. With the thinner insulation, one avoids adjusting the roof with only a small roof overhang (which in old buildings is sometimes not present at all).
A budget of €150,000 for 180 sqm is, in my opinion, not achievable with external contractors. You actually have to work on everything yourself and cannot renovate and leave out some things.

How is the fixed interest period to be understood with the KFW loan?

KfW always specifies fixed interest/fixed term. So 10/10 means 10 years fixed interest and 10 years total term. With possibly 2 grace years without repayment, the loan has to be fully repaid in the last 8 years (i.e. over 8%). In return, you get a better interest rate.
10/30 then means 10 years fixed interest with 30 years total term. So 2.5-3% repayment.

- Do I understand correctly that a clinker brick solid building might have the advantage of cheap "injection insulation"?

No, not necessarily. In that construction year, it was common to build without an air layer. Aside from that, injected insulation may be quite cheap, but usually only involves a few centimeters. Certainly very good, but mostly not sufficient for an 85 standard house.