As for the gameplay itself, Ultima Online seems more of a sandbox, but how much so?
Total Sandbox! There is no end game, your options are limitless!

Can I be a 'healer' class?
Sure can! Just go to the advanced tab when you create and choose healing as one of your options
What is the best server to start on as a new player?
Lake Austin has a very nice dynamic I think. Its not over populated and UOF is there!
Are there any recommended classes for beginners?
Personally I like warrior and you can have healing on there!

UO is a really dynamic game and it takes a little getting used to at first, don't get discouraged. You just need to find a nice guild, and you will find some people really enjoy helping out new people and can be very generous! You can do anything UO, tame dragons, make weapons & armor. everything you use in the game you can make a character to create it. Play around with your available character slots and get a feel for what you like best and we all will be glad to help you with any questions you have!

Good Luck!

~Snuggs