Most missions for all of the expansions, including the basic cities are solo'able. You'll want to make sure you have some source of sneak and invisible, most people use ninja or dancer, sub or main. It's all the same. They're more usable because they don't cause magic agro like the sneak and invisble spells from whm, rdm and sch.
The best advice I'd say would be to get yourself to 99 first, skill up and get some gear. Then you access to missions will be easier.