Yes, I have also bought a lot of tools which I never needed before in my life. And like you, I'm also usually going for the lower end price-wise because chances are that I will not ever need them again after our reno. And frankly, most of the time the tools are completely fine for me.

Interestingly, there already were a number of times where a tool came in handy that I had bought as some point in the past not when I had a need for it at that point but because it was on sale. And if not, I could always resell it, although I probably won't because it's not worth it.

As for the concrete grinder, I actually looked into buying one coming from the same mind set. But the kind of machine that I could rent for 100 bucks costs North of $3000 if I bought it new. And I will likely never need it again after using it once. So this time, I'm surely not going to buy one.

With the windows, I know there's quite a difference in terms of installation depending on where you live. For instance, in the US, you basically buy a window with a flange on it, put it in the whole and screw it into the studs. Then you foam spray the gaps and trim the window. In Germany, installing a window (to code, mind you) is a whole production. Even just measuring what kind of window to order is non-trivial. And I'm sure there's lots of variations in other countries as well.

The thing is Germany, that the quality work is nowadays rare. There is good chance, that the windows will be installed the way you described, the edges plastered and no way to control what's done. German procedure is called "RAL Montage" for it, looks nice in theory, but it takes too long for average "professional". I would say, that picking any window with tripple glas is okay anywhere in Europe. And then comes next problem - the insulation becomes too good and ventilation system must be installed to avoid moisture and mould.

The tools... I think, this is not my last construction site. I must buy some smaller property for my retirement, a house for old people is too much.

I cannot say anything about Germany but in my experience, broad statements like that are to be taken with a grain of salt. There's good workers and there's bad workers, but the old adage "they don't build it like they used to" is often baloney. The quality of most building products (including windows) is much higher today compared to, say, 50 years ago, and as you've probably experienced yourself during your reno, it's not like the quality of work was especially high in the olden days. Less skilled workers without much pride in their craft have always cut corners and done shabby jobs, then and now.

But there definitely are good craftsmen out there, and generally the education they receive today is much higher than one or two generations ago. But, unfortunately, it's true that "you get what you pay for", which makes high quality work often pretty much unaffordable for many. That's the big reason I'm getting myself dirty at our house every day.

There are some jobs that I can do at least as good as hired labor, if not better, but there are other jobs where a qualified worker will give me a better result in the end. You gotta pick your battles.

I would say for single professional you'll find 5-8 below average and max 2 average handymen. The entry barrier is very low and pay ridiculously high. Some examples: stairs were installed and all the cables extended in very amateurish way, but the main grounding cable was cut. Stairs never properly mounted, rather hanging in place by accident. Every extension of wiring ever done is also bad joke. Some original wiring was never replaced. Tiles in one bathroom are bad joke, the lines in the corners are at least by one inch off. Water lines installed destroying half of the brick wall. Lintels basically hanging in the air. 3 glass windows were drilled through to install manual blinds. Nothing dramatic, everything is repairable, but everything was done to make some quick money and must be re-done proper way. Destroyed wall parts rebuilt, proper lintels installed, whole electrical system renewed (I am electrician too).

The single scary thing is the roof. I fall, I die (best case). So I will hire professionals for it.

Edit: some numbers. A guy plastered whole house inside and outside. Took 18000€ in cash and wrote a bill for another 18000€. That’s 5-6 months engineer’s salary. Plastering took 3 weeks. I had many offers for week’s task asking my whole monthly salary in cash. This kind of money attracts not only artisans, but also all possible scammers.