At minimum most setup need at least a Protein Skimmer, Powerheads for circulation, good quality aragonite sand, and some kind of High output lighting in a spectrum you find appealing
1st question ...
Are you planning to use live rock to use live rock in this setup?
This usually comes down to a budget issue in most cases because cured live rock can be very expensive as part of an initial setup
you can get dried live rock as well ... its a fraction of the cost and provides the same benefit to your setup as cured live rock but it doesn't look the same obviously
If youre not going to use any liverock then I would invest in a sand bed filter or wet/dry sump to help break down nitrates and make you're life a lot easier
After that most other equipment you get is just to perform a task to make the tank easier to maintain ( ie: ATO's, controllers, probes, etc )
Something else to think about ... What kind of fish did you want to keep?
Most would consider a 45g a Nano tank and they are kinda small for a non reef / fish only tanks ...
In the size tank you have you will be very limited as to what kind of fish you "should" put in it unless you completely disregard the recommendations on the amount of space the fish need ... some tank raised or smaller fish might be a good fit but most saltwater fish are wild caught and will quickly outgrow your tank as they don't grow to the size of the container you put them in
In almost every instance involving saltwater the tank itself it the smallest investment in the whole setup so if you are going to put together a completely new setup you may want to evaluate the size of the tank if you were thinking bout keeping larger fish ( ie: tangs or angels )