Cannot recommend in glowing terms unless you are an ardent military enthusiast and even so there is not much to see. The Stalin line is mostly in ruins, a pillbox here, a bunker there, and the assorted military equipment on display looks for the most part like it should be on offer at a flea market or just sold for scrap. Anachronistically, much of the equipment on display has nothing to do with Stalin-Line-Era tanks and aircraft. Before the Bel currency crisis, I paid 24000 BYR for entry as a foreigner, 18000 BYR for a non-Belarusian CIS citizen and 12000 BYR for a Belarusian citizen (here strangely they are adhering to three tier price discrimination) but count on paying about triple that now. Then of course renting a taxi for a day trip back and forth (Minsk to Zastavl and return) plus wait time cost around 100$. At least the weather was good (no rain) on the day we went. (As all exhibits are outside, good weather is essential.) I didn't see any reason to hire a guide as the site and the equipment speak for itself. I used the website stalin-line.by (incorrectly listed in the 1st Belarusian Bradt Guide at least) but I found that at the time unfortunately the English part of the website was not functioning. I did glean some useful info from the Russian part of the website, however. (Note that the Stalin Line is closed Mondays.)
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.