Welcome to the forum, Zeemax. (Have I been around long enough to be welcoming people?)
Ideally, you need a planer and a thicknesser. Then to machine a piece of rough sawn timber you firstly cut it to the length you require plus a bit. Then make one face perfectly flat on the planer. You can then machine the opposite face parallel to it on the thicknesser down to the correct thickness.
Back on the planer you plane one edge flat and square to one of the machined faces, and finally machine the opposite edge parallel to the planed edge, normally on the table saw followed by a couple of passes on the thicknesser to clean it up.
You now have a perfectly square, flat, smooth piece of timber to the correct dimensions once you have trimmed it to length.
Fortunately, you don't need two machines (although that would be ideal.) There are many planer/thicknessers available which do both jobs. I am sure you will be inundated with advice as to the best one to get.
For me, the important thing is how easy & quick it is to change between planer and thicknesser modes.
And yes, it would be a worthy investment for the future.
Cheers
Brad