A good cleanser, serum and moisturiser. Plus a good foundation, blusher, eyeshadow and lipstick (though I would focus on just the foundation as skincare should be the priority).
Cleanser- I recommend that you hot cloth cleanse, so invest in a shed load of flannels and a cream cleanser (Superdrug's Vit E is great or Elemis Pro Radiance).
Serum- Estee Lauder Advanced Night Repair, or Olay 3 point serum
Moisturiser- anything from The Body Shop, REN, Elemis or Olay (various budgets there)
Foundation- depends what finish you require. I am prone to dehydration and dullness, so like something a little dewy (but not greasy). Dior Nude skin is the shizzle!
Other makeup bits can be sourced cheaply (check out MUA at Superdrug for a good UD Naked dupe). A decent blusher helps (NARS Deep Throat is good for adding peachiness, or something from Benefit). Lipsticks can vary in price- i don't necessarily think that pricey ones are worth it (I tend to bite my lips a lot, so a waste). Just keep them in good nick with a lip balm (Carmex tinted balms are a good dupe for Clinique Almost lipsticks)
BTW- I have a 3 year old and 1 year old, so don't have a great deal of time for a long winded skin care and makeup routine. Keep hydrated from the inside by avoiding caffeine etc etc and take a fish oil supplement. I have heard good things about applying rosehip oil at night, so might invest in a bottle of that soon...
Enjoy your skincare and makeup overhaul!!
Also, have a look on ebay for bargains