We don't have a good answer for how to start investing in poland. We do have good answers for the more general case, which should also work in Poland. E.g. Best way to start investing, for a young person just starting their career?
This answer provides a checklist of things to do. Let's see how you're doing:
Match on work pension plan.
You don't mention this. May not apply in Poland, but ask around in case it does. Given your income, you should be doing this if it's available.
You have plenty. Either six months of spending or six months of income. Make sure that you maintain this. Don't let us talk you into putting all your money in better long term investments.
High interest debt.
You don't have any. Keep up the good work.
Avoid PMI on mortgage.
As I understand it, you don't have a mortgage. If you did, you should probably pay it off. Not sure if PMI is an issue in Poland.
Not sure if this is an issue in Poland. A personal retirement account in the US.
A reminder to max out whatever your work pension plan allows. The name here is specific to the United States. You should be doing this in whatever form is available.
After that, I disagree with the options. I also disagree with the order a bit, but the basic idea is sound: one time opportunities; emergency savings; eliminate debt; maximize retirement savings. Check with a tax accountant so as not to make easily avoidable tax mistakes.
You can use some of the additional money for things like real estate or a business. Try to keep under 20% for each. But if you don't want to worry about that kind of stuff, it's not that important. There's a certain amount of effort to maintain either of those options. If you don't want to put in the effort to do that, it makes sense not to do this.
If you have additional money split the bulk of it between stock and bond index funds. You want to maintain a mix between about 70/30 and 75/25 stocks to bonds. The index funds should be based on broad indexes. They probably should be European wide for the most part, although for stocks you might put 10% or so in a Polish fund and another 15% in a true international fund.
Think over your retirement plans. Where do you want to live? In your current apartment? In a different apartment in the same city? In one of the places where you inherited property? Somewhere else entirely? Also, do you like to vacation in that same place?
Consider buying a place in the appropriate location now (or keeping the one you have if it's one of the inherited properties). You can always rent it out until then. Many realtors are willing to handle the details for you.
If the place that you want to retire also works for vacations, consider short term rentals of a place that you buy. Then you can reserve your vacation times while having rentals pay for maintenance the rest of the year.
As to the stuff that you have now:
- various collections of books, gold coins, stuff like that, which I guess are worth something also
Look that over and see if you want any of it. You also might check if there are any other family members that might be interested. E.g. cousins, aunts, uncles, etc. If not, you can probably sell it to a professional company that handles estate sales. Make sure that they clear out any junk along with the valuable stuff.
Consider keeping furniture for now. Sometimes it can help sell a property.
- 2 cars (each worth over $60,000)
You might check if you want to drive either of them. If not, the same applies, check family first. Otherwise, someone will buy them, perhaps on consignment (they sell for a commission rather than buying and reselling).
There's no hurry to sell these. Think over whether you might want them.
- 2 large houses and 2 large apartments (every estate is in different major city)
Consider if they hold any sentimental value to you or someone else. If not, sell them. If there's any difficulty finding a buyer, consider renting them out. You can also rent them out if you want time to make a decision. Don't leave them empty too long. There's maintenance that may need done, e.g. heat to keep water from freezing in the pipes.
- a large sum of money (a bit above $250,000)
That's easy, just invest that.
I wouldn't get in too much of a hurry to donate to charity. You can always do that later. And try to donate anonymously if you can. Donating often leads to spam, where they try to get you to donate more.