I used to have this pony! I got her second hand as well. I think my friend hung on to her and our other G1's when her girls "outgrew" ponies, sadly she gave all of the G3's away. BUT - for her hair:
I am fairly certain I just used water, TONS of conditioner, and a pony or similar "doll" brush. Of course start at the ends of the mane and tail and work your way up. I'm sure some of that fluffy hair came out with the detangling, but those ponies have so much of it, you'd never know after it's all brushed out. I was always at a loss as to what to do with her hair after it was detangled, I had somehow never seen a commercial for the perfume ponies and didn't know that you're supposed to just roll with the poofiness (pre-internet times; couldn't just look it up - crazy!). I believe I ended up twisting her mane into little buns down her neck similar to knob braids without braiding first. I think I just braided the tail into a bunch of little braids and tied them together at the ends. Not how you would want to display one if you're interested in authenticity I'm sure, but it did look relatively decent. The mane style reminded me of carousel or show horses. Best of luck and have fun! I'd love to see an "after" photo!