Building a kit can take time and it never finishes! Make up is a personal preferance to you. Make sure you test everything before you buy! Some people will only have mac or benifit etc in their kits but I like to have a good mixture! Its all trial and error. A make up artist may recommend a foundation to you and you dont like it! So good luck and enjoy having a play with all the products out there
This is something only you as a developing artist can decide on, only 4 years later am I happy with my kit but I still add new and reccommended products weekly just to try them and see if they make it into my kit! I worked out to date Ive spent about £4,000 on it gulp :S thats without taking into account what ive spent on things that didnt get into my kit!!! But to begin with just work out what your essentails are and build from there, I would say your biggest and most important investment will be your foundations as they are my base for perfection if the skin is perfect most things seem to follow.
Depending on what area you want to work in you will need different products it really is trial and error.
Pretty much anything that is used or handled that day will have to be either cleaned (if it is possible to clean the item) or otherwise disposables will have to be used. A MUA will also often have to scrap out the products into a pallet - which can lead to more product being used, plus the cost of a pallet, hence part of the reason why a MUA kit can really add up.