Hi Liz,
welcome
I have a bit of experience with ScotlandsPeople.
It is very comprehensive - but unfortunately not cheap!
You have to buy a bank of credits, which then last for a period of time.
The thing I find particularly annoying is that they charge to even view a list of search results from an index, then charge again to view the actual record

.
Anyway, sorry to say that it is easy to get sucked into paying for more and more credits before you know it!
Best of luck with your research, If you can try and tie down as much info as you can before you go on to Scotlandspeople, then you don't end up paying to search, if you know what I mean!
Bye for now,
Hawkeye.