Good for you for having a day to yourself!
Tell the massage therapist about your neck and your sensitivity on your legs/feet. Tell her you aren't sure how you will react. You may find that you are relaxed enough that she can massage other parts of you, but it is okay to tell her you need to play that by ear. Also, with the oils/lotions they use, sometimes their hands move more easily over those sensitive areas, and also a firmer touch often doesn't tickle like a light touch. If she's good, her hands should find the muscles that are tight and which need work. My friend is an extremely experienced therapist, and she says her hands just tell her where to go.
I would expect her to do your head/scalp as well as your neck, shoulders, arms, back, etc. It's all part of the same muscle system. Sometimes they work a little on your jaw if they detect tension there. Just so you know.
Tipping is usually 15-20% but there are different practices when someone is the owner. Some owners don't expect tips - those prices are what they expect. I would make sure she is the owner rather than the person filling in on a Sunday! It may sound awkward, but you can ask - say you don't want to offend her, but you don't want to shortchange her either.
I've not heard of people getting sick - I think that sometimes toxins can be released after a massage, which is good - you've got to get rid of that. She will probably tell you to drink extra water afterwards. It wouldn't hurt to do that for a day or so before also. Just not right before - you don't want to have a full bladder and have to get up during the massage!
Also, don't feel compelled to chat during the massage - it's supposed to be relaxing, and the silence is restorative. She will probably play some soothing music so just relax and enjoy without making a lot of conversation!
Enjoy your day!