I think Abbey would be a good doll to use. Based on your drawing, your OC and Abbey have a similar facial structure. I don't think she will look like a mary-sue. The only thing is you will have to add horns and a tail. I've heard of people doing this with clay, but I don't know how well it would work. For the tail, you could search around eBay is see if anyone is selling a spare tail from one of the cat dolls. You may be able to attach it to Abbey. You'd have to paint it though, so be prepared for that.
Are there any specific questions you have about doing a custom MH doll?
Abbey is arguably the best mold to start a troll girl on. Don't bother taking her glitter off, just cover the body in automotive vinyl dye the color you want your OC to be, the little bits of glitter may give her skin a slight texture, which would be awesome for a troll. Use the right adhesion promoters for the soft vinyl head and hard plastic bodies and apply the dye in two or three light coats, and you'll find yourself with a doll that looks like Abbey, but without the blue sparkly skin. :3
That's probably a good decision. A full repaint is a pain. I just did two and I don't plan on doing any more.
I use Derwent watercolor pencils. They aren't the cheapest, but they are high quality, which matters when doing face-ups. I think you can get a set of 24 for $20 on Amazon though, and that comes with a nice selection of colors. I really like them, and I know a lot of professional face-up artists use them too. I here another good brand is Primsacolor, but I've never used their watercolor pencils. I think they cost about the same as Derwent and are of the same quality.