99's are great little machines and pretty straightforward in design, I vote for doing it yourself, too. I'm not particularly experienced (yet!) but a gummed-up 99 was one of my very first sewing machine "repairs" and the feeling of accomplishment when she went from completely frozen to sewing like a champ was wonderful!
If you're worried about something you're taking apart, just have your camera nearby and take pictures as you take each piece off. That way you'll have a roadmap to putting it back together. And if you get stuck, this forum is super helpful. Lots of 99's in the world so probably at least half of us have one or two, if not more!
On recommendation of this board, I've started using Tri-Flow oil and grease and the machines seem to really love it. I can't say how it will hold up over a decade or so yet but I'll let you know when I get there. LOL