Generally speaking and with a few exceptions neither souvenir sheets nor regular stamps of the past 75 years have much investment value, but they are fun to collect so that is why you should buy them. If you want to buy something that has a chance of increasing in value you should decide what country or countries you want to specialize in, buy the reference books for that country or countries and make yourself an expert to be able to tell the forgeries from the real ones then buy the early and rare stamps from the 1800s that are in perfect condition.
You would also want to become a member of one or more major stamp collecting societies and read their magazines and books and chat with their members. For example, in the September, 2018 American Philatelic Society Magazine there was an excellent article on the enormous number of modern forgeries of souvenir sheets and popular topic stamps of more than 100 countries around the world - mostly in the Caribbean, African and Middle East countries but also many from Asian and Asian Pacific countries as well. Sellers on
ebay offer these all the time and do not tell people they are buying worthless forgeries. You need to become your own expert if you are going to spend more and a few dollars collecting stamps of the past 75 years, and also if you are going to be collecting higher value stamps of the earlier years.
Stamp collecting is primarily something you can do to have fun and learn about other countries and their history.
If you want to invest your money to make a long term profit a better investment is something like the stock markets with an investment horizon of 10 to 20 years so you can ignore the temporary ups and downs.