It sounds to me like you have a very responsible approach to parenting. So many people bring children into the world these days without even giving it thought and they dont have the skills and/or finances to raise their child properly.
I strongly believe that nurture plays a greater role than nature. My mother has mental health problems, and so do I (I have a diagnosis of borderline personality disorder and depression). However I dont believe my issues were simply "passed down" in my genes. I think the fact that my mother was abusive was where my mental health problems stem from, and that's a totally seperate issue to her genuine illness which she cannot help.
I can understand your worries if you know there's a 50/50 chance any children you have may have Tourettes. No one wants to knowingly allow their child to have any health problems. Children are very accepting though and all sorts of people have children for example, people who are in wheelchairs, and so they should! You can explain your tics etc to your child/ren in a way that they understand. If you are able to be a good parent, I dont think your issues on their own will make you a bad parent. Some positive things can come of it too, like your child/ren will learn to be accepting of people who have problems. As you say yourself, you manage okay.
Im 23 but I think I would like children in the future when I am ready with the right housing etc etc. I have done a job where I care for others for 6 years, and in my current job I work with children. I know that I am competant in looking after others and for someone to write me off without taking into account everything would be pure ignorance and predujice.
My current partner has Asperger's, and he functions well like I do with my problems. If we stay together and have a child obviously I would be devasted if they inherited any of our problems. Having said that though, if a child is going to have a mental illness surely it is better they our born to parents who know about it and are understanding. I've had to work with the effects of parents not wanting or being able to cope with children with disabilities etc. I would never give away my child just because he or she had health problems.
Good luck with whatver you decide.