The sciences at Wellesley are very strong: our science center has state-of-the-art equipment for undergrad research, and most science majors work in a lab related to their area of study. If there's a class you want to take that's unavailable at Wellesley, you can cross-register at MIT. I'd encourage you to check out the course catalogue, as well as different departments' websites: Course Catalog | Wellesley College - Wellesley College
In regards to graduate school, I can give you statistics I've learned as a student admissions representative. ~66% of Wellesley students who apply to grad school are accepted to their first choice school. 80% of all Wellesley graduates eventually gain a postgraduate degree. Our medical school acceptance rate is around 75% (the national average for other schools is 45%). Generally speaking, most universities know that Wellesley is a rigorous college and understand that its graduates are able to handle difficult graduate work.
If it's possible, you should consider doing an overnight visit at Wellesley and attending a couple of classes. If you have any questions, just shoot me a PM!