If you've never ridden English before an all-purpose saddle will be just fine. I wouldn't get anything really expensive since its just your starting saddle. Look for something thats well-priced, comfortable, fits your horse and is USED. You won't have to break it in, and it'll be cheaper, but still in great shape. I would suggest one with knee rolls. Since you're used to riding western, your leg is probably a bit forward when you ride and knee rolls will help push your leg back a bit, plus I find they are comfy! :P
Your saddle may or may not come with stirrup leathers and irons, so make sure you get those (try to match leathers colour with saddle colour). Since it is your first time riding English, I would strongly suggest getting safety stirrups. (They have a little elastic in place of metal on one side so if you should fall, your foot will not get caught. The elastic will snap off and your foot will be free).
You'll also need:
- a cinch (girth) ... your saddle may not come with one
>> I prefer one with stretchy at both ends where the buckles are (measure your horse's cinch area so it fits her well)
- saddle pad (an all-purpose one will do)
- a bit (probably a regular snaffle: o-ring or d-ring)
- well-fitting English bridle (preferably matching the colour of your saddle, but doesn't really matter)
- matching reins (they come in two pieces but attach in the middle)
Here's an example of what English tack looks like:
http://www.horsecountrydiscount.com/images/1417comp.jpg(this one is brand-new but it has the knee-rolls, snaffle bit, etc).
Try maybe getting a package like in the picture, then you get the whole shebang! Good luck and happy birthday!