To answer your two easy questions go for a fibreglass boat if you want to mimize maintenance, and go for fixed keel so it less likely to break if you run aground.
if you are really keen a $4k boat might be ok to start with since they are smaller and easy to handle. if you go mad and just buy one theres plenty of people who will come out on it with you and teach you.
In light wind you can go out and teach yourself even, just pull sail down and use motor to get out of trouble if you see it coming. Week days are best for that since waterways have less traffic.
Learning on other peoples boats will teach you a lot but its not the same as being skipper and being in control. if you want your own boat you must know about safety principles, what the winds doing, what sails to put up and how to use them. Also you will start to learn about the maintenance side as mentioned above. Being skipper will force all that on you. My opinions different to the above, i would buy one and get a teacher.
Also you can use pc sailing simulators to learn. I got my gf who never sailed in her life to practice heaps on one and when we got onto boat she knew how to sail upwind reach run and tack instantly