Stam led the Royals to third place in his first season in charge and they now face a play-off meeting with Fulham.
The Dutchman believes the Championship will be a stronger division next season with some big-spending clubs.
"If you want to make steps yourself as a club, you don't want to sell your best players," he said.
"Sometimes you can have things too quick, but if the opportunity's there to do things well, you need to enjoy it and that's what we're doing."
Reading and Fulham meet in the first leg of their semi-final at Craven Cottage on Saturday before the return leg at the Madejski Stadium on Tuesday.
Back in August, former Manchester United centre-back Stam felt finishing in the Championship top 10 would be a success for Reading.
But, now the club find themselves three games away from a return to the top flight for the first time since 2013.
"You're always worried if you have a squad with good players that they're going to go," the 44-year-old admitted.
"This season, a lot of players have stepped up, they've shown themselves and there's been a lot of interest from other clubs in the league.
"So, it's very important that if someone goes and I understand at times clubs need money, that you can spend that money as well.
"With the clubs coming down to the Championship and those who've stayed down and shown intention to invest, the top 10 if you look at it money-wise, it's going to be very difficult to get into the top six for the teams who are not able to spend a lot of money."
Reading manager Jaap Stam fears losing some of his best players should they miss out on promotion to the Premier League through the play-offs.