1) There are many, many posts on this site from people who own older i-devices such as your iPhone who upgraded the iOS software to the latest and then discover they have to buy a new computer because theirs cannot upgrade the OSX software to run the newest iTunes iOS9 demands. If you buy a new phone it will require that newer software and you will have to buy a newer computer or never sync your phone directly to your computer.
2) Keep using your MacBook. You decide what you need. Until last week I was using a 2002 G4 with OSX 10.4.11. I don't have any i-gadgets and even now under Mavericks I still use iTunes 7.5. If all you need is a newer browser then use Firefox (I prefer it over Safari anyway because it has lots of nice extensions). On my G4 I used TenFourFox which is up to date.
My wife has an iPhone 5s but has never upgraded iOS, so it is happy with her old iTunes. It is really kind of up to you if you want to join the Apple software upgrade rat race. Eventually when you do get a newer iPhone, then you will be forced to have a newer computer unless you do as my wife does which is never syncs her phone to iTunes (even though she can with her computer now). Back when we only had my G4 she had an iPhone 4s and it was impossible to sync, but it didn't matter because she never plays media on her phone.
Apple will recycle your MacBook. For them it is too old to sell. If you want to get money for it you are better off selling it yourself, but realize that because it has that ceiling it has kind of reached a breakpoint. I was glad to get my current late 2008 MacBook because it could still run new generation software and was worth the hardware upgrades I did (8GB RAM, new, big, fast hard drive).