Don't skimp here...
Mate, you will need to do the front as well.
A well-sprung rear with a soggy front is not a recipe for a god time. And the front is only sprung for a 70kg rider.
Seriously, the biggest payback you will get for your bucks is getting the suspension sorted, springs AND damping, by a real expert. Added safety, much more comfortable, way better handling, less fatigue (you're not wrestling the bike on rough stuff), blah blah blah. Just do it.