Most builders will use MDF and in my experiences owner building it's probably the best option. When you consider the cost of MDF at around $1 per lineal meter versus Hardwood at say $10, then when you're ordering hundreads of meters of skirting and arc then the costs will rise very quickly. Also, by the time MDF is gloss painted it's got a moisture barrier to a certain degree, and its easier to sand and finish versus working with hardwood. So it's cheaper, easier to work and provides pretty much the same option.
If you're concerned about it warping in wet areas like bathrooms and laundries, then again the gloss paint should provide adequate moisture barrier to stop water ingress and warping. Although if you're urging on the side of caution then you could always use hardwood in these rooms. Remembering if you're skirts are getting water damaged then it's likely you'll have bigger issues with cabinetry and subfloor rot, so skirts and arc will be the least of your issues.
I hope this helps.