I will give you the best advice I can and I hope everyone here agrees. Every pinball is going to have problems. Most problems are maintenance related. Taking the machine apart and cleaning everything will help immensely. While your cleaning you will find almost all of your broken or damaged parts. Spend the cash and replace them. Don't cheap out as you will usually be taking it back apart again to fix the same part again. Take lost of pics, ask lots of questions, and take your time.
I have had old Bally machines that have been a bigger pain in the ass then STTNG. If you like the game then buy it.