L.,
This is a very personal decision, with no right or wrong answers. My children started at age 2, 2 mornings a week, 2 1/2 hrs each morning; it was fantastic for them, and if I had to do it all over again, I would.
Preschool is not about academics (some are, but I would stay away from those "push down academics" situations) but about social skills and development.
Your daughter gets to be at home with you, which is wonderful; preschool meets other needs, and you probably can find one where you don't have to send your child too many hours a week, if you want.
As for not wanting to be left alone unless it is family, this is all part of the learning experiences of a child (and of her mom!). When my daughter started preschool, she just said "Bye, Mommy! See you later!" when I was trying to figure out how to tell her I was leaving! That is not the norm, though. Many kids cry at drop off for a few days, and then, one day, they just don't cry and can't get into their preschool classes fast enough! My son cried the first few times I dropped him off...sometimes because he saw other kids crying...but even so, from the very first day, he told me he couldn't wait to go back!
My kids went to an amazing preschool, so nurturing and so aimed at social skills and so not into the academics.
I have very bright children who are in gifted support, but I was very careful not to send them to a place that emphasized academics. That's what kindergarten/elementary school is for, as far as I'm concerned! I gave them those "academic" skills at home when they were of preschool age--reading street signs, doing basic math as we walked down the street and saw things, writing the alphabet, etc.--and I never saw the need to have a preschool teach these things to my child.
The kindergarten teacher that both my kids had at public school once told me that she likes to get the kids from our preschool because they come in with such great social skills. That's what it's all about!
If you do look for a preschool, look for one that feels really good to you, of course! Look at the artwork on the wall, too. Is it too perfect to have been created 100% by the kids? If so, I might not want that teacher.... a preschool director once said that to me, and as I took her advice and looked at the art work, I could see a difference in teacher styles, even within the same school.
My son is 10 and some of his very best friends still are ones he met when he was 2 years old. They are almost like family. He has never spent a day with them in public school, but they still attend each others’ birthday parties, get together on weekends, etc. They formed such strong bonds…all the way back when they were 2 and 3 years old!
Good luck!
(Oh, depending on the preschool, and on your child, it may feel better to start your child in the preschool's camp program. Our preschool's summer camp program is very different from the school year program, and I did start my daughter in the camp.)