But adding memory to most Macs is a doddle. I've upgraded both our machines to maximum using chips from thes guys : Other World Computing (OWC) - Performance Upgrades For Your Mac. (Satisfied customer speaking, no other relationship with company) Their website has videos showing how to install the chips, so you can decide in advance if it's something you want to do.
Caveat: You might want to do a bit of research on upgrading RAM on Apples laptops. With older laptops it was possible to add RAM from third party sources.
Today's Apple laptops have RAM soldered in. I just had a quick look at their site and unless I have missed something I believe that if you want more RAM than offered as standard you must specify that when you order what you want.
I believe that the only models that you can add other than Apple RAM are the 27" iMac and the Mac Pro.
I second the suggestion to max the RAM to your budget limits.
FWIW I have a 2007 MBP 15" that still works well and can run Yosemite, not the fastest but it still works well.